Hapalosiphon intricatus has been reported to produce an extracellular substance that inhibits the growth of another blue-green alga, an Anabaena sp. A similar antialgal response has been reported for H. fontinalis toward A. oscillarioides. In neither case has the active principle been identified. In our work H. fontinalis was isolated from soil samples collected in the Marshall Islands in 1981. The lipophilic extract of the cultured alga showed antialgal and antimycotic activities. An edaphic form of Hapalosiphon fontinalis (Ag.) Bornet (Stigonemataceae), strain number V-3- 1, was isolated by repeated subculture on solidified media. The alga was cultured in 25-L glass bottles containing an inorganic medium modified by replacing citrate buffer with 3 mM 3-morpholinopropanesulfonic acid (pH 7) and by supplementing the medium with a comprehensive minor and trace element mixture. Cultures were illuminated continuously at an incident intensity of 330 FEinstein m-, s-I from banks of cool-white fluorescent tubes. Cultures were vigorously aerated with 1% COz in air and incubated at 24 f 1 OC. The alga was harvested by filtration; yields typically were 0.445-g dry weight of cells per liter of culture. The freeze-dried alga (360 g) was extracted with 1:l i-PrOH/CHZCl2 and the oily extract (15.1 g) was subjected to gel filtration on Sephadex LH-20 with 1 : 1 i-PrOH/CH,Cl, and rapid chromatography on silica gel (TLC grade) with hexane, 1: 1 hexane/CHzClz, CH2Cl2, CH2Cl2/EtOAc, EtOAc, and EtOAc/EtOH. The fraction that was eluted with 1:l hexane/CHzC12 was purified by HPLC on Whatman Partisil with 1:l hexane/CH2C12 to give 2.1 g (0.58%) of hapalindole A (l), mp 160-167 OC dec and [alZsD -78'(CHZClz, c 1.2) after crystallization from hexane/CH,Cl2 and sublimation at 125 OC (0.1 mm). Hapalindole A was responsible for most of the antialgal and antimycotic activity of H. fontinalis. Compound 1 exhibited a UV spectrum [A,, (e) 222 (38 000), 280 (7000), 291 nm (5800)l typical of an indole and IR and I3C NMR peaks [Y,,, 2145 cm-'; 613~ 157.401 characteristic of an isonitrile. A molecular formula of C21H23N2C1 could be deduced for the alkaloid from detailed analyses of the 'H and 13C NMR spectra (Table I) and a high-resolution E1 mass spectrum (M' observed at m/z 338.1595; mmu error 4.6).