Developing embryos of the inland silverside fish, Menidia beryllina, were exposed to conidiospores of the insect pathogenic fungus, Beauveria bassiana, that possessed activity against the migratory grasshopper, Melanoplus sanguinipes. Various adverse effects were observed in Menidia beryllina embryos and larvae. They included rupture of the chorion, embryo death, developmental defects (vertebral abnormalities) in the embryo or hatched larvae, and fungal infections on the mandibles of larvae. Although there was little evidence of a definitive dose-response trend based on densities of viable conidiospores, statistically significant (p < or = 0.01) responses were observed in tests in which conidiospore densities were as low as 7.1 x 10(3)/ml and as high as 1.3 x 10(6)/ml. Viable spores were required for adverse effects to occur; heat-killed spores failed to cause significant adverse effects.