ZnSe epitaxial layers have been grown for the first time on (100) GaAs substrates by metalorganic molecular beam epitaxy using diethylzinc and diethylselenide. Diethylselenide is cracked by the cracking furnace. Smooth single-crystalline layers can be grown at substrate temperatures above 200 degree C. Photoluminescence spectra vary noticeably with the substrate temperature. Above 400 degree C, the spectra measured at 71 K were dominated by intense blue donor-acceptor pair bands, and broad luminescence bands at longer wavelengths were weak.