The dynamics of transmission of disease agents by vectors depends, in part, on the probability of host-vector contact, which can vary with fluctuations of both host and vector. As important as seasonal variations is 24-hour periodicity in activity. Periodicity in the landing of males and females of Aedes aegypti on man has been assessed by means of catches of 15 hours or longer, with several persons as a bait. The assessments were made in a suburban area of Tanzania and continued throughout one year. Activity was observed to be almost entirely diurnal and diphasic. Whereas the detailed activity pattern of males agreed closely with that found elsewhere in East Africa, that of the females was unusual on account of the symmetry of the morning and afternoon peaks. Possible causes of differences among studies are discussed.