Can I pass Externalizable objects by value using RMI, or do the objects have to implement Serializable directly?

RMI will allow you to pass any object that is serializable according to the serialization specification. This includes objects that implement Externalizable as well as objects that implement Serializable.

Note that Externalizable extends Serializable, so anywhere you see the use of the generic term "serializable" you can assume it refers to objects that implement either of these interfaces.