The title compound, [1aS]-6-amino-8{[(aminocarbonyl)oxy]methyl}-1,1a,2,8,8a,8b- hexahydro-8a-methoxy-5-(phenylthiomethyl)azirino[2′,3′:3,4]pyrrolo[1,2-a]indole- 4,7-dione, C21H22N4O5S, is a C6-substituted methyl mitomycin C and which possesses potent antitumor activities. The N4 atom is more pyramidal than the corresponding atom in mitomycin C anhydride and mitomycin C dihydrate.