The sponge Reniera sp. contains renierone (l), mimosamycin (3), N-formyl- 1,2-dihydrorenierone (7), O-demethylrenierone (8), 1,6-dimethyl-7-methoxy-5,8-dihydroisoquinoline-5,8-dione (9), 2,5-dimethyl-6-methoxy-4,7-dihydroisoindole-4,7-dione (10), and renieramycins A-D (1 1-14). Mimosamycin (3) was first isolated from Streptomyces lavendulae no. 314, which also contained mimosin (2) and the saframycins (4-6), compounds similar in structure to renierone (1) and the renieramycins (11-14). The structure of 2,5-dimethyl-6-methoxy-4,7-dihydroisoindole-4,7-dione (lo), the first naturally-occurring isoindole to be described, was confirmed by an unambiguous synthesis. The structures of the renieramycins were determined by analysis of spectral data, particularly the 'H NMR data.