Malformin B, produced by Aspergillus niger, was separated into six compounds by HPLC. Their structures were determined by amino acid analyses, and by mass spectral and two-dimensional NMR experiments to be cyclic pentapeptides structurally related to malformin A1. Both the NMR and MS/MS experiments suggest cyclo-D-cysteinyl-D-cysteinyl-L-amino acid-D-amino acid-L-amino acid as the essential structure of malformins.