A study of the sporogonic cycle of the gibbon malaria parasite, Plasmodium jefferyi, has demonstrated the comparative susceptibility of four anopheline mosquitoes, Anopheles balabacensis balabacensis, A. freeborni, A. maculatus, and A. quadrimaculatus, and has delineated the length of the cycle and the morphology of the several sporogonic stages. Anopheles b. balabacensis showed the highest rates and intensity of infection. With the exception of the presence of prominent sporoblasts in A. freeborni, the sporogonic stages of the parasite were similar in all mosquito species used. Oocysts ranged in diameter from 8 ,u on day 5 (the earliest day of observation) to 57 A, on day 13. Sporozoite-positive glands were first seen on day 13 (A. b. balabacensis), day 15 (A. freeborni), or day 17 (A. maculatus). No positive glands were found in the A. quadrimaculatus. Plasmodium jefferyi was described from a Hylobates lar in West Malaysia by Warren et al. (1966) and is one of four malaria parasites which have been found naturally in gibbons. Because of the scarcity of gibbons as laboratory animals, little subsequent work has been possible with this parasite and only recently, with the availability of a small number of gibbons for such work, has it been possible to study the sporogonic stages. Through these investigations some observations can now be presented on the infectivity of P. jefferyi to four species of Anopheles, including information on the sporogonic cycle in these mosquitoes. The morphologic characteristics of the sporogonic stages of the parasite have been determined and are illustrated with photomicrographs. MATERIALS AND METHODS The donor animal (H. lar No. 42.0) was infected at the Delta Regional Primate Research Center in Covington, Louisiana. The details of the origin of this infection are presented elsewhere (Coatney et al., 1969). During the course of the infection, mosquitoes were sent by air from our Laboratory in Chamblee, Georgia to the Delta Regional Primate Research Center. The different mosquito species were fed simultaneously on the animal and, after feeding, were returned, also by air. The total travel time did not exceed 30 hr. Upon return, the mosquitoes were held in a low-temperature incubator at 25 ? 1 C for the remainder of the extrinsic incubation period. Mosquitoes were fed 5% Karo solution daily on a cellulose pledget. Beginning at day 5 and continuing through day 14, sample mosquitoes were removed from * Delta Regional Primate Research Center, Covington, Louisiana 70433. Received for publication July 25, 1969. the cages, dissected, and examined for the presence of occysts on the gut. The oocysts, if present, were measured with an ocular micrometer using 430 X magnification. Representative oocysts were photographed after measurement was completed. Beginning at 13 and continuing through 17 days after infection, sample mosquitoes were dissected and examined for the presence of sporozoites in the salivary glands. The intensities of the gland infections were rated 1+ to 4+ (Collins et al., 1966). Mosquitoes used were all derived from colonies maintained in our Laboratory. The A. balabacensis balabacensis colony was obtained from the Walter Reed Army Institute of Research, Washington, D. C., and originated in Thailand (Esah and Scanlon, 1966). The A. freeborni were originally from Marysville, California, and have been maintained in the Laboratory for many years (Hardman, 1947). The A. maculatus were obtained from the Institute for Medical Research, Kuala Lumpur, Malaysia (Ow Yang et al., 1963). The A. quadrimaculatus were the Q-1 strain from the southeastern United States and were obtained from Technical Development Laboratories, NCDC, Savannah, Georgia.