The reservoir of infectious Plasmodium falciparum gametocytes in a population living in an area of holoendemic malaria in western Kenya was estimated by directly feeding mosquitoes on volunteers. Resulting mosquito infections were assessed both by midgut examination for oocysts and by enzyme-linked immunosorbent assay for P. falciparum circumsporozoite antigen. Calculations based on the age structure of the population and the resulting rates of mosquito infections indicated that children under 10 years of age were responsible for 72% of mosquito infections, individuals between 10 and 21 years of age contributed 12%, and those over 21 years of age accounted for 16%. No infection resulted in mosquitoes fed on infants less than 1 year of age.