Bioassay-guided fractionation of a non-polar extract of Lyngbya cf. polychroa resulted in the isolation of the cytotoxic desacetylmicrocolin B (1) as well as the known compounds microcolins A (2) and B (3). Compound 1 was found to inhibit the growth of HT-29 colorectal adenocarcinoma and IMR-32 neuroblastoma cells with half maximal inhibitory concentration (IC(50)) values of 14 nM for both cancer cell types. Microcolins A and B were found to have little activity against two strains of the marine fungus Dendryphiella salina with LD(50) values above 200 microg/mL. Compounds 1, 2, and 3 were obtained by reverse-phase chromatography and their structures were determined by NMR and MS. In this paper we report the isolation, identification, and biological activity of 1.