Results are presented which strongly suggest that strains of Aspergillus fumigatus are one of the predominant fungi in molded corn silage. The clavine alkaloids, fumigaclavine A, a new alkaloid designated fumigaclavine C, and several tremorgens belonging to the fumitremorgen group were produced by A. fumigatus strains isolated from molded silage. The LDW of fumigaclavine C was about 150 mg/kg oral dose in day-old cockerels. Calves dosed with crude extracts of A. fumigatus cultures experienced severe diarrhea, irritability, loss of appetite, and postmortem examination showed serous enteritis and evidence of interstitial changes in the lungs; abnormal changes were not found in other tissues. We have investigated a naturally occurring acute toxic syndrome in beef cattle of unknown etiology. The syndrome was characterized by a general deterioration of the herd which was typical of protein deficiency and malnutrition, resulting in some mortalities even though ample pasture and supplemental feed were available. Other clinical signs were diarrhea, irritability, and abnormal behavior. Necropsy of a cow and calf showed that both were emaciated. There were signs of previous Ostertagia in the abomasum of the cow. The only other gross abnormality was a large (8411. diameter) ulcer in the wall of the rumen of the calf. Evaluation of the rumen ulcer indicated that both a fungus and bacterium were involved in its development. Histopathological examination showed that the hepatic parenchymal cells of both animals had a relatively clear cytoplasm suggestive of protein depletion. The cattle involved were being fed corn silage, from a trench-type silo, that had molded because the packed silage face had been exposed to the atmosphere for 1-2 weeks before emptying and feeding. The silo was too wide for the size of the herd and improper emptying and feeding practices were used. The major fungus isolated from the molded silage was identified as Aspergillus fumigatus (AFu-4). We now report the toxin-producing potential of six isolates of A. fumigatus from several sources. Three isolates (AFu-3, AFu-4, AFu-5) represent the dominant fungi from two silos associated with toxic syndromes of cattle and one (AFu-6) was from an unused silo. The isolation and identification of the major toxins produced by these fungi are described. Preliminary data on the effects of crude extracts of one A. fumigatus isolate (AFu-3) on test cattle are presented.