Infection experiments with 3 species of bird malaria (Plasmodium eathemerium Hartman; P. inconstans Hartman; and P. praecox Grassi and Feletti) and several species of culicine mosquitoes showed that the following mosquitoes are susceptible to all 3 species of parasites: Culex salinarius Coq., C. territans Walker, and C. pipiens Linn. Species found to be susceptible to infection with the first 2 of the above parasites were Aedes aegypti Linn. and C. quinquefasciatus Say. Species found to be insusceptible to the first 2 species of parasites were Aedes triseriatus Say, A. canadensis Theob., A. sollicitans Walker, A. vexans Meig., A. cantator Coq., and Anopheles punctipennis Say; and, in addition, Psorophora sayi D. & K. was found to be insusceptible to P. eathemerium. Complete transmissions were effected with P. cathemerium by C. pipiens and salinarius, and with P. inconstans by C. pipiens. Studies on immunity in the mosquito brought forth evidence of an individual immunity in a susceptible species (C. pipiens). The length of life of the asexual forms of the parasite (eathemerium) in the stomachs of C. pipiens and A. sollicitans was 6-6 hrs. in each case. Ookinetes were formed in the stomachs of each of these mosquitoes in 12 hrs. Digestion of red blood corpuscles required about the same amount of time in each of these 2 species of mosquitoes. The hypothesis that differences in digestion in C. pipiens and A. sollicitans plays an important ro1e in immunity and susceptibility of mosquitoes is abandoned. Observations on the parasites showed significant differences in the prepatent periods of all 3 parasites, and that no periodicity occurs in the gametocytes over a 24-hr. period.