Thank you very much for your replies, Helen and Thomas!

> Alternatively, have your on-site installer install the correct client
> version into the system32 directory. Always check the property sheet
> of your gds32.dll file to verify that you have the correct version
> for the server.

This is only alternative, when it's only one db engine used on one
machine (InterBase only or FireBird only), right?

I have to run apps with access to both engines on the same machine, so
i have only the first possibility (app path based library), right?

> Everything goes fine, except i have installed IB7 engine on the
> machine, then firebird library won't run from app path and even from
> system path.

And what about my "mystery", please? Have you any ideas, please?