A vehicle-mounted net was used to study the circadian flight activity of several species of Simulium in a northern Guinea savanna area in Nigeria during the dry season. The sampling method yielded large numbers of both sexes of Simulium squamosum (Endertein) of the, S. damnosum Theobald complex, S. hargreavesi Gibbins, S. vorax Pomeroy, S. adersi Pomeroy, S. hirsutum Pomeroy and other species. The main peak of activity of all species recorded occurred just after sunset and there was a smaller peak just before sunrise. Flies continued to be caught at a low level 2·5 h after sunset when sampling ceased. Differences in the activity patterns of S. squamosum males and females and of the other species were analysed. Of the S. squamosum females caught, 12% were blood-fed; these and gravid females were mainly active in the evening, while the blood-thirsty flies had a high level of activity throughout the day. Differences between species in the relative activity of blood-thirsty and gravid flies, and nulliparous and parous flies are noted.