Jim-

>I'm getting the error message "File '58070939.dbf' does not exist." on the ALTER TABLE statement below; I don't understand how a file created by "COPY TO" could not exist, especially after I have USE'd it. (When the error occurs, the file is open in the current work area).

I can't duplicate the problem. You mention your user is on NT, does this mean you don't see the problem on XP but the user does on NT? Anyway, just a WAG, you might try not using the table before altering the columns. I was sorta surprised it even works to use it first since AFAIK, ALTER TABLE works on the disk file.