The preference of Culex quinquefasciatus Say for oviposition in rearing water (in which larvae and pupae of the same species had been reared), pond water, tap water (from a well) deionised water and food water (in which larval food was mixed) was tested in a 5 × 5 Latin square. Most oviposition occurred in rearing water, followed in decreasing order by pond water, tap water, deionised water and food water, the differences being significant except between pond water and tap water. Diel oviposition activity of females from wild-caught pupae was studied under tropical normal (light-day) photoperiod (LD 12:12), reversed (dark-day) photoperiod (DL 12:12) and continuous light (LL) for at least three consecutive days at average conditions of 29·3°C and 91% RH. Under conditions of alternate light and dark periods (normal as well as reversed), there was a circadian rhythm, which was unimodal, confined to the scotophase and concentrated in the first half of the scotophase. Under continuous light, activity was greatly depressed, with disturbances both in circadian rhythm and unimodality. It is concluded that the circadian rhythm of oviposition activity of C. quinquefasciatus is predominantly exogenous in nature and regulated mainly by photoperiod, activity being stimulated by darkness and inhibited by light.