showed a significantly greater number of larvae than the refractory genotype at days 5 and 8 (t = 4.536, P < 0.001 at 5 days; ROCK = 0 L3's at 8 days). Our results indicate that a susceptible mosquito genotype will maintain a higher infection (88%) and support a greater number of larvae (overall mean= 13.06) than a refractory genotype (44% susceptible; mean larvae = 9.76). Both genotypes initially show high infections (BLACK EYE = 100% susceptible; ROCK = 88%). However, significant differences in filarial development occur 2-3 days post-infection, with the refractory genotype demonstrating a much greater decrease in susceptibility and number of developing larvae. Eight days post-infection several larvae will approach complete development in a susceptible genotype. By contrast, most larvae are eliminated by the fifth day in a refractory genotype (Table I). Similar changes in filarial development were also obtained by Morison (1973, M.S. Thesis, Univ. Texas Schl. Publ. Hlth, Houston, Texas) and by Rodriguez et al. (unpubl. data) when specific susceptible and refractory genotypes of Ae. aegypti were infected with B. pahangi. The authors thank Prof. George B. Craig, Jr., Director of the WHO International Reference Centre for Aedes, University of Notre Dame, for supplying the BLACK EYE and ROCK strains. The strain of B. malayi was from infected jirds obtained from Dr. John McCall, University of Georgia through the U.S.-Japan Co-operative Medical Science Program-NIAID. This study was supported in part by NIH Research Grant No. RR-08194-04.