Beauveria bassiana spores and metabolites were evaluated for toxicity and pathogenicity to Mysidopsis bahia. Static acute 96-hr tests were conducted with < or - 24-h-old M. bahia using either conidiospores, the mycotoxin, beauvericin, or a nonpolar extract of the mycelia. Conidiospore densities of > or - 1.5 X 10(6)/ml caused high mortalities. These mortalities were attributed to a high particulate density since heat-killed controls also proved lethal. Beauvericin, a cyclic depsipeptide produced by some strains of B. bassiana, was toxic at an LC50 of 0.56 mg/l. The toxicity of beauvericin persisted in sterile seawater for at least three, but not eight weeks. A nonpolar extract of mycelia from B. bassiana containing beauvericin was toxic at an LC50 of 84.2 mg/l. In contrast, a nonpolar extract of mycelia from the fungal weed pathogen, Colletotrichum gloeosporioides f. sp. aeschynomene (Cga), was not toxic when tested up to 70.4 mg/l.