The toxigenicity of 392 strains ofAspergillus, representing 132 species and 19 varieties, was assessed on chicks and mice fed iso-caloric 50 % basal feed mixes of wheat and soybeans molded by these strains. On the basis of death numbers (3 or more of 6 on test), low weight gain of survivors, and low feed consumption over a 4-week period, 166 of the strains, representing 73 species and 9 varieties, were toxigenic. Ten species (A. alliaceus, A. avenaceus, A. clavato-flavus, A. janus, A. melleus, A. ochraceus, A. parasiticus, A. quercinus, A. sclerotiorum, andA. sulphureus) had strains that were markedly toxic to both animal types on one or both feeds; the other 63 species and 9 varieties had strains that were moderately to mildly toxic to one or both animal types on one or both feeds. In retests of some strains, toxigenicity varied between successive preparation of molded feed.