House flies (Musca domestica L.) were permitted to emerge, mature, and mate in an opaque polyethylene enclosure, free from light leakage detectable by Kodak Royal PanR film. The film (ASA speed number 400) was continuously subject to exposure within the test enclosure. Consummation of mating in darkness (minimum range 2500–6500 A) was demonstrated by successful oviposition and subsequent development of a second generation of flies with normal sex ratios.