Bioassay-guided fractionation of the 7:3 MeOH/water extract of a cultured cyanobacterium strain identified as Fischerella sp. yielded nine isonitrile-containing alkaloids. Three of the compounds, ambiguine H isonitrile (1), ambiguine I isonitrile (2), and ambiguine J isonitrile (3), are new, while the other six, 12-epi-hapalindole H, ambiguine A isonitrile, ambiguine B isonitrile, ambiguine D isonitrile, ambiguine E isonitrile, and ambiguine F isonitrile, have been previously isolated from Fischerella ambigua. The structures of the compounds were determined by 1D and 2D NMR techniques and mass spectrometric data. Ambiguine H isonirile and ambiguine I isonirile possess antibacterial and antimycotic activity.