Yes, an file with an mtl extention is a file that carries the material information (e.g. texture/colour) for a file with the same name but with an obj extention. It must live in the same directory as its associated obj (Waveront Object)file. Poser does not care where obj files live UNLESS they are associated with(linked from)a pp2 or cr2 file. You need to keep the mtl file (unless you resave it as a pp2), otherwise the object won't display the correct materials. Poser 4 does not have a any set place to put obj files that are not associated with cr2 (character), or pp2 (prop) files. I create a sibdirectory under Poser 4 to put them in.