I'm not an authority, just a voracious reader. On the back of the diopter, if there is an M embossed it should fit the K10D - if not it probably won't. The +2 diopter I just bought is for my KX (same size as the K1000) so I can try it tonight on the K10D and PM you.

Will the -4 Diopter Eyepiece for Pentax K1000 fit on the K10D?