Analysis by a progeny test system has permitted the selection for different cytoplasmic sterility factors in two strains of mosquitoes derived from the progeny of a single female. Reciprocal interstrain crosses show full fertility and viability. However, testcrosses to a third cytoplasmic type show highly significant, reproducible differences in the cytoplasmic systems of the selected strains. A direct relationship between teratological growth patterns and the degree of heterozygosity of cytoplasmic factors was observed. In testcrosses, cytoplasmic sterility factors, when selected to homozygosity, produce non-teratological inviable haploid, or exceptional viable gynogenetic diploid larvae. Heterozygosity in the cytoplasmic system in testcrosses produced a broad spectrum of teratological growth patterns related to the degree of heterozygosity of the multiple factors present in the cytoplasm. Formal genetic crosses involving the selected strains show that teratological growth and cytoplasmic sterility, both partial and complete, are inherited through the germ plasm of the maternal parent. This work suggests that the interactions of cytoplasmic factors that result in teratological growth and cytoplasmic sterility may not be restricted to Culex pipiens, where in some cases it is prominent and obvious, but may be a much more general hereditary mechanism of major significance in other eukaryotic animals.