Mosquitoes were fed at all stages of infection on 88 cases of Plasmodium falciparum. Observations were made on the relations of gametocyte densities and length of patency to infectivity. Mosquitoes frequently became infected when fed on gametocyte densities of less than 10 per cmm. of blood, and were infected as late as day 321 of parasite patency in the South Carolina strain, and 410 in the Panama strain. It is concluded that the long enduring parasitemias of these South Carolina and Panama strains of P. falciparum are of considerable epidemiological importance and may be responsible for a large part of the transmission of this species in certain endemic areas.