A chemical toxicity/teratogenicity test was adapted to assess potential adverse effect of a microbial pest control agent on a nontarget fish. Developing embryos of inland silverside, Menidia beryllina, were exposed to conidiospores of the insect-pathogenic fungus, Beauveria bassiana. Adherence of spores to the chorion followed by germination and penetration by the germ tube caused the embryos to rupture, sometimes resulting in death. Conidiospores treated with a biological detergent showed significantly less binding (p < or = 0.01) to embryos than did untreated spores. Both detergent-treated and heat-killed spores failed to cause significant pathogenic responses.