diff --git a/js/source/worker.js b/js/source/worker.js
index 841984379d6..ecfbb5899e2 100644
--- a/js/source/worker.js
+++ b/js/source/worker.js
@@ -15,29 +15,28 @@ function Worker(self) {
     this.self = self;
     this.actor = new Actor(self, this);
 
-    // simple accessor object for passing to WorkerSources
-    var styleLayers = {
-        getLayers: function () { return this.layers; }.bind(this),
-        getLayerFamilies: function () { return this.layerFamilies; }.bind(this)
-    };
+    this.layers = {};
+    this.layerFamilies = {};
 
-    this.workerSources = {
-        vector: new VectorTileWorkerSource(this.actor, styleLayers),
-        geojson: new GeoJSONWorkerSource(this.actor, styleLayers)
+    this.workerSourceTypes = {
+        vector: VectorTileWorkerSource,
+        geojson: GeoJSONWorkerSource
     };
 
+    // [mapId][sourceType] => worker source instance
+    this.workerSources = {};
+
     this.self.registerWorkerSource = function (name, WorkerSource) {
-        if (this.workerSources[name]) {
+        if (this.workerSourceTypes[name]) {
             throw new Error('Worker source with name "' + name + '" already registered.');
         }
-        this.workerSources[name] = new WorkerSource(this.actor, styleLayers);
+        this.workerSourceTypes[name] = WorkerSource;
     }.bind(this);
 }
 
 util.extend(Worker.prototype, {
-    'set layers': function(layers) {
-        this.layers = {};
-        var that = this;
+    'set layers': function(mapId, layers) {
+        var styleLayers = this.layers[mapId] = {};
 
         // Filter layers and create an id -> layer map
         var childLayerIndicies = [];
@@ -60,20 +59,21 @@ util.extend(Worker.prototype, {
         function setLayer(serializedLayer) {
             var styleLayer = StyleLayer.create(
                 serializedLayer,
-                serializedLayer.ref && that.layers[serializedLayer.ref]
+                serializedLayer.ref && styleLayers[serializedLayer.ref]
             );
             styleLayer.updatePaintTransitions({}, {transition: false});
-            that.layers[styleLayer.id] = styleLayer;
+            styleLayers[styleLayer.id] = styleLayer;
         }
 
-        this.layerFamilies = createLayerFamilies(this.layers);
+        this.layerFamilies[mapId] = createLayerFamilies(this.layers[mapId]);
     },
 
-    'update layers': function(layers) {
-        var that = this;
+    'update layers': function(mapId, layers) {
         var id;
         var layer;
 
+        var styleLayers = this.layers[mapId];
+
         // Update ref parents
         for (id in layers) {
             layer = layers[id];
@@ -87,41 +87,41 @@ util.extend(Worker.prototype, {
         }
 
         function updateLayer(layer) {
-            var refLayer = that.layers[layer.ref];
-            if (that.layers[layer.id]) {
-                that.layers[layer.id].set(layer, refLayer);
+            var refLayer = styleLayers[layer.ref];
+            if (styleLayers[layer.id]) {
+                styleLayers[layer.id].set(layer, refLayer);
             } else {
-                that.layers[layer.id] = StyleLayer.create(layer, refLayer);
+                styleLayers[layer.id] = StyleLayer.create(layer, refLayer);
             }
-            that.layers[layer.id].updatePaintTransitions({}, {transition: false});
+            styleLayers[layer.id].updatePaintTransitions({}, {transition: false});
         }
 
-        this.layerFamilies = createLayerFamilies(this.layers);
+        this.layerFamilies[mapId] = createLayerFamilies(this.layers[mapId]);
     },
 
-    'load tile': function(params, callback) {
+    'load tile': function(mapId, params, callback) {
         var type = params.type || 'vector';
-        this.workerSources[type].loadTile(params, callback);
+        this.getWorkerSource(mapId, type).loadTile(params, callback);
     },
 
-    'reload tile': function(params, callback) {
+    'reload tile': function(mapId, params, callback) {
         var type = params.type || 'vector';
-        this.workerSources[type].reloadTile(params, callback);
+        this.getWorkerSource(mapId, type).reloadTile(params, callback);
     },
 
-    'abort tile': function(params) {
+    'abort tile': function(mapId, params) {
         var type = params.type || 'vector';
-        this.workerSources[type].abortTile(params);
+        this.getWorkerSource(mapId, type).abortTile(params);
     },
 
-    'remove tile': function(params) {
+    'remove tile': function(mapId, params) {
         var type = params.type || 'vector';
-        this.workerSources[type].removeTile(params);
+        this.getWorkerSource(mapId, type).removeTile(params);
     },
 
-    'redo placement': function(params, callback) {
+    'redo placement': function(mapId, params, callback) {
         var type = params.type || 'vector';
-        this.workerSources[type].redoPlacement(params, callback);
+        this.getWorkerSource(mapId, type).redoPlacement(params, callback);
     },
 
     /**
@@ -130,13 +130,28 @@ util.extend(Worker.prototype, {
      * function taking `(name, workerSourceObject)`.
      *  @private
      */
-    'load worker source': function(params, callback) {
+    'load worker source': function(map, params, callback) {
         try {
             this.self.importScripts(params.url);
             callback();
         } catch (e) {
             callback(e);
         }
+    },
+
+    getWorkerSource: function(mapId, type) {
+        if (!this.workerSources[mapId])
+            this.workerSources[mapId] = {};
+        if (!this.workerSources[mapId][type]) {
+            // simple accessor object for passing to WorkerSources
+            var styleLayers = {
+                getLayers: function () { return this.layers[mapId]; }.bind(this),
+                getLayerFamilies: function () { return this.layerFamilies[mapId]; }.bind(this)
+            };
+            this.workerSources[mapId][type] = new this.workerSourceTypes[type](this.actor, styleLayers);
+        }
+
+        return this.workerSources[mapId][type];
     }
 });
 
diff --git a/js/style/style.js b/js/style/style.js
index 9641d24825c..6ca6481c572 100644
--- a/js/style/style.js
+++ b/js/style/style.js
@@ -731,7 +731,7 @@ Style.prototype = util.inherit(Evented, {
 
     // Callbacks from web workers
 
-    'get sprite json': function(params, callback) {
+    'get sprite json': function(_, params, callback) {
         var sprite = this.sprite;
         if (sprite.loaded()) {
             callback(null, { sprite: sprite.data, retina: sprite.retina });
@@ -742,7 +742,7 @@ Style.prototype = util.inherit(Evented, {
         }
     },
 
-    'get icons': function(params, callback) {
+    'get icons': function(_, params, callback) {
         var sprite = this.sprite;
         var spriteAtlas = this.spriteAtlas;
         if (sprite.loaded()) {
@@ -756,7 +756,7 @@ Style.prototype = util.inherit(Evented, {
         }
     },
 
-    'get glyphs': function(params, callback) {
+    'get glyphs': function(_, params, callback) {
         var stacks = params.stacks,
             remaining = Object.keys(stacks).length,
             allGlyphs = {};
diff --git a/js/util/actor.js b/js/util/actor.js
index dc09086cfeb..0c0fd9da40c 100644
--- a/js/util/actor.js
+++ b/js/util/actor.js
@@ -10,11 +10,13 @@ module.exports = Actor;
  *
  * @param {WebWorker} target
  * @param {WebWorker} parent
+ * @param {string|number} mapId A unique identifier for the Map instance using this Actor.
  * @private
  */
-function Actor(target, parent) {
+function Actor(target, parent, mapId) {
     this.target = target;
     this.parent = parent;
+    this.mapId = mapId;
     this.callbacks = {};
     this.callbackID = 0;
     this.receive = this.receive.bind(this);
@@ -32,17 +34,19 @@ Actor.prototype.receive = function(message) {
         if (callback) callback(data.error || null, data.data);
     } else if (typeof data.id !== 'undefined' && this.parent[data.type]) {
         // data.type == 'load tile', 'remove tile', etc.
-        this.parent[data.type](data.data, done.bind(this));
-    } else if (typeof data.id !== 'undefined' && this.parent.workerSources) {
+        this.parent[data.type](data.mapId, data.data, done.bind(this));
+    } else if (typeof data.id !== 'undefined' && this.parent.getWorkerSource) {
         // data.type == sourcetype.method
         var keys = data.type.split('.');
-        this.parent.workerSources[keys[0]][keys[1]](data.data, done.bind(this));
+        var workerSource = this.parent.getWorkerSource(data.mapId, keys[0]);
+        workerSource[keys[1]](data.data, done.bind(this));
     } else {
         this.parent[data.type](data.data);
     }
 
     function done(err, data, buffers) {
         this.postMessage({
+            mapId: this.mapId,
             type: '<response>',
             id: String(id),
             error: err ? String(err) : null,
@@ -54,7 +58,7 @@ Actor.prototype.receive = function(message) {
 Actor.prototype.send = function(type, data, callback, buffers) {
     var id = null;
     if (callback) this.callbacks[id = this.callbackID++] = callback;
-    this.postMessage({ type: type, id: String(id), data: data }, buffers);
+    this.postMessage({ mapId: this.mapId, type: type, id: String(id), data: data }, buffers);
 };
 
 /**
diff --git a/js/util/dispatcher.js b/js/util/dispatcher.js
index 7ca9bd7c367..cc7a9206e9b 100644
--- a/js/util/dispatcher.js
+++ b/js/util/dispatcher.js
@@ -16,9 +16,10 @@ module.exports = Dispatcher;
 function Dispatcher(length, parent) {
     this.actors = [];
     this.currentActor = 0;
+    this.id = util.uniqueId();
     for (var i = 0; i < length; i++) {
         var worker = new WebWorker();
-        var actor = new Actor(worker, parent);
+        var actor = new Actor(worker, parent, this.id);
         actor.name = "Worker " + i;
         this.actors.push(actor);
     }