diff --git a/src/ExternalModule.js b/src/ExternalModule.js
index e8749f8..757d56d 100644
--- a/src/ExternalModule.js
+++ b/src/ExternalModule.js
@@ -1,18 +1,19 @@
+import { blank } from './utils/object';
+import makeLegalIdentifier from './utils/makeLegalIdentifier';
 
 export default class ExternalModule {
 	constructor ( { id, bundle } ) {
 		this.id = id;
 
 		// Implement `Identifier` interface.
-		this.originalName = id;
-		this.name = id;
+		this.originalName = this.name = makeLegalIdentifier( id );
 		this.module = this;
 
 		// Define the external module's name in the bundle scope.
 		bundle.scope.define( id, this );
 
 		this.isExternal = true;
-		this.importedByBundle = [];
+		this.importedByBundle = blank();
 
 		// Invariant: needsNamed and needsAll are never both true at once.
 		// Because an import with both a namespace and named import is invalid:
diff --git a/src/Module.js b/src/Module.js
index deef71b..d10eaa7 100644
--- a/src/Module.js
+++ b/src/Module.js
@@ -224,6 +224,7 @@ export default class Module {
 			if ( isNamespace ) {
 				// If it's a namespace import, we bind the localName to the module itself.
 				module.needsAll = true;
+				module.name = localName;
 				this.locals.bind( localName, module );
 			} else {
 				const name = isDefault ? 'default' : specifier.imported.name;
@@ -237,6 +238,7 @@ export default class Module {
 				if ( module.isExternal && isDefault ) {
 					const id = module.exports.lookup( name );
 					module.name = id.name = localName;
+					id.name += '__default';
 				}
 			}
 		});
@@ -393,10 +395,16 @@ export default class Module {
 	mark ( name ) {
 		const id = this.locals.lookup( name );
 
-		if ( id && id.statement ) {
-			// Assert that statement is defined. It isn't for external modules.
-			id.isUsed = true;
-			id.statement.mark();
+		if ( id && id.module ) {
+			if ( id.module.isExternal ) {
+				id.module.importedByBundle[ id.originalName ] = true;
+			}
+
+			if ( id.statement ) {
+				// Assert that statement is defined. It isn't for external modules.
+				id.isUsed = true;
+				id.statement.mark();
+			}
 		}
 	}
 
@@ -601,7 +609,11 @@ export default class Module {
 					.forEach( name => {
 						const id = this.locals.lookup( name );
 
-						if ( id.module && id.module.isExternal ) {
+						// We shouldn't create a replacement for `id` if
+						//   1. `id` is a Global, in which case it has no module property
+						//   2. `id.module` isn't external, which means we have direct access
+						//   3. `id` is its own module, in the case of namespace imports
+						if ( id.module && id.module.isExternal && id.module !== id ) {
 							replacements[ name ] = id.originalName === 'default' ?
 								// default names are always directly accessed
 								id.name :
diff --git a/src/utils/makeLegalIdentifier.js b/src/utils/makeLegalIdentifier.js
index 7e0a846..1dd4454 100644
--- a/src/utils/makeLegalIdentifier.js
+++ b/src/utils/makeLegalIdentifier.js
@@ -8,7 +8,10 @@ reservedWords.concat( builtins ).forEach( word => blacklisted[ word ] = true );
 
 
 export default function makeLegalIdentifier ( str ) {
-	str = str.replace( /[^$_a-zA-Z0-9]/g, '_' );
+	str = str
+		.replace( /-(\w)/g, ( _, letter ) => letter.toUpperCase() )
+		.replace( /[^$_a-zA-Z0-9]/g, '_' );
+
 	if ( /\d/.test( str[0] ) || blacklisted[ str ] ) str = `_${str}`;
 
 	return str;
diff --git a/test/form/external-imports/_expected/cjs.js b/test/form/external-imports/_expected/cjs.js
index ac9f79d..b6f5d31 100644
--- a/test/form/external-imports/_expected/cjs.js
+++ b/test/form/external-imports/_expected/cjs.js
@@ -1,10 +1,10 @@
 'use strict';
 
 var factory = require('factory');
-factory = 'default' in factory ? factory['default'] : factory;
 var baz = require('baz');
 var containers = require('shipping-port');
 var alphabet = require('alphabet');
+factory = 'default' in factory ? factory['default'] : factory;
 var alphabet__default = 'default' in alphabet ? alphabet['default'] : alphabet;
 
 factory( null );
diff --git a/test/form/external-imports/_expected/es6.js b/test/form/external-imports/_expected/es6.js
index cd41111..4d05b46 100644
--- a/test/form/external-imports/_expected/es6.js
+++ b/test/form/external-imports/_expected/es6.js
@@ -1,10 +1,10 @@
 import factory from 'factory';
 import { bar, foo } from 'baz';
 import * as containers from 'shipping-port';
-import alphabet, { a } from 'alphabet';
+import alphabet__default, { a } from 'alphabet';
 
 factory( null );
 foo( bar );
 containers.forEach( console.log, console );
 console.log( a );
-console.log( alphabet.length );
+console.log( alphabet__default.length );