When A. quadrimaculatus mosquitoes were fed on rabbits infected with D. uniformis, the percentage of anophelines surviving decreased with increasing intensity of microfilarial infection. Increases in the size of the infective dose, up to 310 microfilariae per 0.02 ml of blood, brought about a decrease in the percentage of infective larvae which developed. Dissections of over 750 mosquitoes harboring a total of more than 15,000 larvae revealed that the distribution of larvae was essentially the same regardless of the size of the infection. Larval filariae collected at 10 days either from the head or from the thorax and abdomen were equally infective for rabbits when injected subcutan-eously. When infected mosquitoes were fed on normal rabbits, the insects lost a considerable number of larvae from the thorax and abdomen as well as from the head. The number of larvae recovered from mosquitoes maintained at 27[degree]C was significantly higher than in those kept at 24[degree], and 31[degree]C. Larvae developing at 31[degree] C grew more rapidly during the first 7 days than those at the lower temperatures, but by the tenth day the larvae at 27[degree] C had attained the greatest mean length. Significantly more numerous and longer larvae developed in mosquitoes 12 to 13 days old than in those 4 to 5 days old. Fewer new larvae developed in super-infected mosquitoes than in previously uninfected controls, but the differences were not statistically significant.