Several mutants of the fungus Gibberella fujikuroi were shown to produce cyclopentane fatty acids of the jasmonic acid type. Detailed investigation of the culture filtrate of one mutant resulted in the isolation and structural elucidation of (+)-7-iso-jasmonic acid and (-)-jasmonic acid as well as their (S)-isoleucine conjugates and 4,5 didehydro-9,10-dihydrojasmonic acid. Out of nine further mutants studied, all of them produced the two isoleucine conjugates but in only four of them could free jasmonic acid be detected (max amount of 2.5 mgl-I).