Results are presented which show that Aspergillus fumigatus was one of the predominant fungi contaminating moldy silage. Growth of A. fumigatus on silage appeared to depend on a preliminary aerobic fermentation by other natural microflora in silage. The clavine alkaloid, fumigaclavine A, and a new clavine alkaloid designated fumigaclavine C were produced by A. fumigatus. The LD50 of fumigaclavine C was approximately 150 mg/kg oral dose in day-old cockerels.