To investigate the quantitative distribution of Plasmodiumfalciparum gametocytes into the vector bloodmeal, Anopheles arabiensis mosquitoes were fed on 3 volunteers naturally infected with gametocytes. The content of each mosquito midgut was smeared on a microscope slide and Giemsa stained. The distribution of gametocytes ingested by mosquitoes followed a negative binomial distribution, with apparently constant overdispersion (parameter k +/- S.E. = 3.105 +/- 0.392) for the 3 series. This aggregation of gametocytes in some midguts probably facilitates the conjunction of gametes and fertilization. This suggests that P. falciparum gametocytes in the peripheral blood flow of infected man do not follow an independent, homogeneous pattern but show a significant aggregation.