Abstract The productivity per female of the Tangmere and Thorne strains of Oryzaephilus surinamensis (L.) was studied at 25 °C, 70% r.h. in continuous dark (DD) and in 12 h cycles of light and dark (LD 12:12), in which the illumination intensity of the photophase was 100–200 lx. Intra- and reciprocal inter-strain crosses indicated that whilst productivity did not differ between the crosses in DD, the productivity of the Thorne females was suppressed in LD 12:12. When the F 1 hybrid progeny, reared under either light condition, were back-crossed with Tangmere in LD 12:12, differences in productivity were not detected, but a heterotic effect was indicated. Whereas photoperiodic conditions did not influence the productivity of the F 1 hybrid progeny, productivity of the Thorne strain was influenced. Adults of this strain cultured under a particular photoperiod were less productive in the alternative photoperiod. An increase in illumination intensity to 700–800 lx suppressed the productivity of additional strains, including the Tangmere strain in LD 12:12. However, an intermediate level of productivity was observed in several of these strains in constant light at this intensity. Therefore, the productivity of the Tangmere and Thorne strains was assessed during the scoto- and photophases at this increased illumination intensity. Preliminary experiments indicated that the disturbance associated with the experimental method also suppressed productivity over a 12-h cycle. Therefore, an artificial photoperiod of LD 24:24 was used to minimise this effect after a 3-week entrainment period. The productivity of both strains was greater in the scotophase than in the photophase and suggests that the suppression of productivity in LD 12:12 was associated with the difference between the phases of this photoperiod.