The effects of sex, age of the human host, patency of asexual and sexual stages and seasonality on infectiousness of Plasmodium falciparum to mosquitoes were investigated in a rural village in southern Tanzania between 1992 and 1994. Villagers from randomized subgroups of households were surveyed for malaria parasites. Gametocyte and trophozoite prevalences were age dependent and fluctuated without any clear pattern of seasonality. A sample of 107 participants, selected to include an excess of gametocyte carriers, slept under bednets with holes cut into the sides for 3 weeks. A total of 3837 Anopheles gambiae s.l. and 5403 A. funestus recovered from these bednets, was examined for all oocysts 5-7 days after feeding or for oocysts less than 17.5 microns in diameter 2-3 days after feeding. Additional blood slides from participants were taken twice weekly. The 5-7 day oocyst rates were 12.1% in A. gambiae s.l. and 10.9% in A. funestus and 2-3 day rates were 3.6 and 4.9%, respectively. The higher rates using the former method were attributed to previous infection. There were strong correlations in the levels of infection in both vectors when they fed on the same hosts. However, patent gametocytaemia was only weakly associated with the development of oocysts in the mosquito. Infectiousness was not related to host age, sex, or the season.