N-Methylwelwitindolinone C isothiocyanate (7), the major indole alkaloid in the blue-green algae (cyanobacteria) Hapalosiphon welwitschii W. & G. S. West UH IC-52-3 and Westiella intricata Borzi UH HT-29-1 (Stigonemataceae), is responsible for some of the multiple-drug-resistance (MDR) reversing activity in H welwitschii and most of the insecticidal activity in W. intricata. The oxindole was isolated from the lipophilic extracts of the two cyanophytes and its structure and absolute stereochemistry determined by X-ray crystallography. Fourteen additional alkaloids were isolated from H. welwitschii as minor constituents, viz. a novel spiro oxindole, welwitindolinone A isonitrile (1), five tetracyclic oxindoles (1-6) related to 7, and eight biogenetically-related fischerindoles (8-11) and hapalindoles (12-15). In addition to 1, five other alkaloids were isonitriles and these six compounds collectively accounted for the fungicidal activity of H. welwitschii. The gross structures and relative stereochemistries of 1-6 and 8-15 were elucidated by spectral analysis. The biogenesis of these alkaloids is discussed.