Fontonamide (3) and anhydrohapaloxindole A (4) are two minor alkaloids that have been isolated from a cultured strain of the terrestrial blue-green alga Hapalosiphon fontinalis. Both compounds appear to be singlet oxygen oxidation products of hapalindole A (1), the major alkaloid in this cyanophyte. Hapalonamide A (5), the probable precursor of fontonamide, is formed along with 3 and 4 when an oxygen-aerated solution of 1 in aqueous methanol buffered at pH 8 with sodium phosphate and containing a trace of rose bengal is irradiated at room temperature. To date, however, 5 has not been identified as a constituent of H. fontinalis. © 1987, American Chemical Society. All rights reserved.