Thanks for your reply. The intended ultimate viewer app is Clarisworks, but I'd be happy with SimpleText. The Japanese displays correctly in Outlook Express. When I save the message to a text file, it displays fine in SimpleText. But after the script appends the message to the archive text file, it's just garbage. I guess I should look at these files with a hex editor, but I don't have one on this machine right now.
OE is presumably using the Content-transfer-encoding header in the message to determine that it's Japanese in the first place. I don't know how it communicates this to SimpleText; probably it adds a resource to the file indicating which script system to use. Maybe that's part of the style information. Appending the contents of the file (i.e. its data fork) would of course omit the resource. You would have add such a resource to the archive file (and no, you can't just copy it from one of the messages).
Another possibility is to script SimpleText or ClarisWorks to paste the text at the end of the file (instead of doing it yourself). Others (such as Chris) would be able to assist you with doing that from MacPerl. (Consult me if you want to do it in C++.)
Josh