Summary and Conclusions Complete mosquito feedings were done on 24 patients with early primary parasitemias of Plasmodium vivax (Chesson strain). The average infection produced infected mosquitoes on the fourth day of demonstrable parasites, two days before gametocytes could be discovered by normal examination of blood smears, two days following the onset of fever (100°F. or above), and about one day previous to the first fever of 104°F. or more. It is concluded that these early parasitemias may be of epidemiological importance, in that, even in endemic areas where good therapeutic measures are available, the primary attack would provide a reservoir for mosquito infection before it could be terminated or suppressed. In indigenous populations of endemic areas it would be unusual for the naturally acquired vivax infection to be terminated or suppressed before the infected individual became a potential source of mosquito infections.