Two wild-type laboratory strains of Drosophila melanogaster were used in this study: strain Flordia-9, which is sensitive to aflatoxin B 1 (AFB 1)-induced toxicity, and strain Lausanne-S, which is resistant. Eggs of these strains were deposited on medium containing either low or high doses of dietary AFG 1, AFB 2, or sterigmatocystin (ST) and allowed to develop into second instar larvae. After this pretreatment, the larvae were transferred onto medium containing either high or low doses of dietary AFB 1 (post-treatment) and allowed to complete development and eclose as adults. Viability and development data were analyzed to determine the effects of the various pretreatments on the level of AFB 1-induced toxicity in the post-treatments. In no case did any of the pretreatments reduce the toxic effects of AFB 1 post-treatment responses. However, for strain Florida-9, all high-dose pretreatments resulted in enhanced post-treatment toxicity, and all low-dose pretreatments also enhanced toxicity of high-dose post-treatments. For strain Lausanne-S, high-dose AFB 2 pretreatment significantly enhanced toxicity of both high- and low-dose post-treatments. These results indicate that, in strain Florida-9, pretreatment with relatively less toxic mycotoxins (ST and AFB 2) has an enhancing effect on AFB 1-induced toxicity, whereas in strain Lausanne-S, a similar but smaller enhancing effect is seen only with AFB 2 pretratment.