The microsclerodermins are cyclic hexapeptides from a deep water sponge of the genus Microscleroderma. The structures of microsclerodermins A (1) and B (2) were determined by interpretation of spectral data. The absolute stereochemistry at each of the 10 asymmetric centers was determined by interpretation of NMR data, particularly the nOe data, and by chemical degradation followed by analysis of the resulting amino-acids by GC-MS using a chiral column. Microsclerodermins A and B inhibit the growth of C. albicans at 2.5 mu g/disk. Examination of the symbiotic filamentous micro-organisms associated with the sponge revealed that they were eubacteria similar in appearance to members of the family Beggiatoaceae.