Two new indolic metabolites, fumitremorgin A and B, were isolated from the fungus Aspergillus fumigatus. These two metabolites caused severe tremor and convulsion in experimental animals. The structure of fumitremorgin A, including the stereochemistry, was determined to be 1 mainly on the basis of the spectral data for the compound and some derivatives obtained from it. The stereochemical factors affecting the appearance of tremorogenic activity in fumitremorgin A and related compounds are discussed.