New melanin synthesis inhibitors (MR566A and B) and six related known isocyanocyclopentenes were isolated from the fermentation broth of Trichoderma harzianum, and their structures were elucidated by spectroscopic methods. The structures of novel isocyanides, MR566A (1) and B (2), were elucidated as 1-(3-chloro-1,2-dihydroxy-4-isocyano-4-cyclopenten-1-yl)etha nol, 1-(1,2,3-trihydroxy-3-isocyano-4-cyclopenten-1-yl)ethanol, respectively. The structure of novel oxazole, MR93B (9), was elucidated as 4-[(1Z)-3-hydroxy-2-hydroxymethyl-1-propen-1-yl]oxazole.