The com.apple.mrj.* package was shipped with OS 9 (and maybe early versions of OS X ?). It's not part of the Java 1.4 that is shipped now, so if OS X is your target, the com.apple.eio.* package is the way to go. But if you're concerned about backwards-compatibility, you can use both, by properly enclosing them in try/catch blocks, and if one fails, use the other.
There's also the MRJAdapter library, which encapsulates this functionality, so your work is done! [ November 28, 2005: Message edited by: Ulf Dittmer ]