diff --git a/src/execution/execute.js b/src/execution/execute.js
index f419ebc156..620e507577 100644
--- a/src/execution/execute.js
+++ b/src/execution/execute.js
@@ -798,7 +798,13 @@ function completeAbstractValue(
   info: GraphQLResolveInfo,
   result: mixed
 ): mixed {
-  const runtimeType = returnType.getObjectType(result, info);
+  let runtimeType: ?GraphQLObjectType;
+  if (returnType.resolveType) {
+    runtimeType = returnType.resolveType(result, info);
+  } else {
+    runtimeType = defaultResolveTypeFn(result, info, returnType);
+  }
+
   if (runtimeType && !returnType.isPossibleType(runtimeType)) {
     throw new GraphQLError(
       `Runtime Object type "${runtimeType}" is not a possible type ` +
@@ -859,6 +865,25 @@ function completeObjectValue(
   return executeFields(exeContext, returnType, result, subFieldASTs);
 }
 
+/**
+ * If a resolveType function is not given, then a default resolve behavior is
+ * used which tests each possible type for the abstract type by calling
+ * isTypeOf for the object being coerced, returning the first type that matches.
+ */
+function defaultResolveTypeFn(
+  value: mixed,
+  info: GraphQLResolveInfo,
+  abstractType: GraphQLAbstractType
+): ?GraphQLObjectType {
+  const possibleTypes = abstractType.getPossibleTypes();
+  for (let i = 0; i < possibleTypes.length; i++) {
+    const type = possibleTypes[i];
+    if (typeof type.isTypeOf === 'function' && type.isTypeOf(value, info)) {
+      return type;
+    }
+  }
+}
+
 /**
  * If a resolve function is not given, then a default resolve behavior is used
  * which takes the property of the source object of the same name as the field
diff --git a/src/type/definition.js b/src/type/definition.js
index 8c360173c0..e0e219fc74 100644
--- a/src/type/definition.js
+++ b/src/type/definition.js
@@ -600,30 +600,11 @@ export class GraphQLInterfaceType {
     return Boolean(possibleTypes[type.name]);
   }
 
-  getObjectType(value: mixed, info: GraphQLResolveInfo): ?GraphQLObjectType {
-    const resolver = this.resolveType;
-    return resolver ? resolver(value, info) : getTypeOf(value, info, this);
-  }
-
   toString(): string {
     return this.name;
   }
 }
 
-function getTypeOf(
-  value: mixed,
-  info: GraphQLResolveInfo,
-  abstractType: GraphQLAbstractType
-): ?GraphQLObjectType {
-  const possibleTypes = abstractType.getPossibleTypes();
-  for (let i = 0; i < possibleTypes.length; i++) {
-    const type = possibleTypes[i];
-    if (typeof type.isTypeOf === 'function' && type.isTypeOf(value, info)) {
-      return type;
-    }
-  }
-}
-
 export type GraphQLInterfaceTypeConfig = {
   name: string,
   fields: GraphQLFieldConfigMapThunk | GraphQLFieldConfigMap,
@@ -721,11 +702,6 @@ export class GraphQLUnionType {
     return possibleTypeNames[type.name] === true;
   }
 
-  getObjectType(value: mixed, info: GraphQLResolveInfo): ?GraphQLObjectType {
-    const resolver = this._typeConfig.resolveType;
-    return resolver ? resolver(value, info) : getTypeOf(value, info, this);
-  }
-
   toString(): string {
     return this.name;
   }