I've recently upgraded to RH9.0. Unfortunately the new default window managers for gnome (metacity)
and kde (kwin) both have problems with one of the image display applications I use. I've established
that my application works fine under twm and mwm, but I've been unable to figure out how to change
the default window manager for either gnome or KDE.

The /usr/bin/startkde file implies that all I have to do is set the environmental variable KDEWM to
what I want, i.e., /usr/X11R6/bin/twm, but when I set this and then start x (either manually via
startx or by the usual automatic procedure via init 5) it doesn't seem to have done anything.
Ditto with changing the environmental variable WINDOW_MANAGER for gnome. Clearly
I'm doing something wrong but I haven't figured out what. So, any help would be appreciated.

I'd also disappointed to see "sawfish" didn't come with the Red Hat 9 installation (despite what
it says in the reference manual). But one thing at a time....

thanks.

When you first boot (or whenever you boot), at the login screen you will see the word "Session". Click on it and you will be able to change the window manager. It will also have the option to set that window manager as the default.

That would work for selecting the desktop session (gnome, say, or kde) but not the underlying window
manager that is being used by these desktops. Thanks for the suggestion, though.

Does anyone know how to do this?

I stumbled on TWM quite by accident once.
I made a note to look at xinitrc to fix it.

Must have worked because I'm running Gnome now...

This link looks informative...
http://www.rapierbit.org/linux/winmgrs.html

Cheers,
alar