A cytotoxic antibiotic, zygosporin A, was isolated from the culture filtrate of Zygosporium masonii and its stereostructure was elucidated as (7€,13€)-(3S,3aR,4S,6S,6aR,I OS.12R.15R.15aR) -1 5-acetoxy-3-benzyl-3.3a.4.5.- 6,6a,9,10,12,15-decahydro-6,12-dihydroxy-4,10.12-trimethyl-5-rnethylenecycloundeca[d]isoindole-1 ,I 1 (2H) - dione (XXXIII) by chemical degradation and X-ray analysis. It has been reported that cytochalasin D is identical with zygosporin A.