Ochratoxin A, a toxic metabolite from Aspergillus ochraceus Wilh. is shown to be 7-carboxy-5-chloro-8-hydroxy-3,4-dihydro-3~-methylisocoumarin, linked over its 7-carboxy-group to L-P-phenylalanine by an amide bond. Two further metabolites, ochratoxins B and C, have been isolated and characterised as the dechloro and ethyl ester derivatives of ochratoxin A, respectively. Full stereochemical forniulz are suggested.