Circadian activity was compared among four strains of Culex pipiens molestus Forskal from Iran, Egypt, two localities (Ebira and Yanagawa) in Nagasaki, Japan, and one strain of Cx. p. pallens Coquillett from Japan. Under a 16:8 (L:D) h photoperiod, all the strains showed peaks of activity at lights on and off, with additional activity during both light and dark periods; Egyptian females and both sexes of Cx. p. pallens were inactive during the photophase. Under 0:24 (L:D) h, the length of the free-running activity period was significantly shorter in the "Iran" (21 h) than in the other three Cx. p. molestus strains (range, 21-23 h) and Cx. p. pallens (approximately 24 h). Under 24:0 (L:D) h, the activity rhythm was obscured. The length of the period was shorter in 24:0 (L:D) h than in 0:24 (L:D) h, violating Aschoff's circadian rule. Furthermore, the circadian peak did not always occur every day under 24:0 (L:D) h but rather exhibited "day-skipping."