Fifteen wild-type laboratory strains of Drosophila melanogaster were tested for egg-adult viability when exposed to larval development in media containing 0.5 and 1.0 ppm aflatoxin B (AFB1). Significant variation among the strains was demonstrated, especially at the 0.5 ppm AFB1 concentration. Two resistant and two sensitive strains were made isogenic and mated in a 4 X 4 diallel system. Results indicate that differences in AFB1 sensitivity are due to genes with additive effects on viability and that nonsystematic effects due to the interaction of cytoplasms and genes of both paternal and maternal origin are present. A chromosome/cytoplasm substitution analysis was performed using one of the sensitive and one of the resistant strains. Results indicate that genes on chromosomes X and 2 contribute to egg-adult viability differences observed for larval growth on media containing 0.5 and 1.0 ppm AFB1. Also, a significant interaction between chromosome 2 and cytoplasm, both from the resistant strain, was observed, resulting in a twofold increase in viability at 0.5 ppm AFB1 when compared to controls. Possible relationships of these findings with those from vertebrate systems are discussed.