Tests with colonies of Culex pipiens fatigans from six localities in Kenya and Tanzania revealed high levels (60-90%) of susceptibility to infection with Wuchereria bancrofti. Twenty-six of 30 possible crosses between these colonies were performed and revealed no cases of incompatibility. All but 2 crosses produced near-normal sex ratios of F(1) adults, with a slight excess of females in 16 of 20 detailed assessments. The implications of these results for control work and filariasis transmission are discussed. From other data it is concluded that the strain of C. p. fatigans present in Kenya and Tanzania is homologous with populations found elsewhere in Africa and in southern Asia.