XML serialization doesn't capture base class elements

I am using Xml serialization to serialize a class to an XML file. This works fine, except that instances of this class are derived from another class, and I need the base class variables to be written to the Xml file too.

Both the class that gets serialized and the base class are marked as serializable.

Is there a way to get this to work?

thanks.

Using .Net 3.5SP1