Fischerindole L (3) is a novel octahydroindeno[2,1-hlindole isonitrile from the terrestrial cyanophyte Fischerella muscicola that possesses the same relative stereochemistry as hapalindole L (4). In evaluating hundreds of laboratory-cultured blue-green algae (cyanobacteria) for antifungal activity, the extract (70% ethanol) of Fischerella muscicola (Thuret) Gomont (UTEX 1829) was found to inhibit the growth of four test fungi, viz. Aspergillus oryzae, Penicillium notatum, Saccharomyces cerevisiae, and Trichophyton mentagrophytes, in a soft-agar disc-diffusion assay (250 pg, lo-17 mm zones). Using a bioassay-guided isolation scheme,1 most of the antifungal activity was associated with an indole alkaloid fraction. Hapalindoles A (1J2 and J (2)x and fontonamide4 were the major components in this mixture by HPLC analysis. In addition a new antifungal5 tetracyclic alkaloid, fischerindole L (3), possessing a hexahydroindeno[2,1+]indole ring system and an isonitrile functionality, and having the same relative stereochemistry as hapalindole L (4),3 was present. We present here the isolation and structure determination of 3. The UV spectrum of 3 was typical of an indole and the EI mass spectrum, coupled with NMR data, established that 3 had the same elemental composition (C21H23ClN2) as 1 and 4.6 Inspection of the lH and 13C NMR spectra in acetone-d& indicated that an o-disubstituted benzenoid ring, a vinyl and three methyl groups attached to quarternary carbons, and a CHeqX-CHeq-CHax-CH2-CHaxY unit in a six-membered ring were present. Since the H-11 (4.36 ppm) and C-11 (63.5 ppm) signals showed a 1: 1: 1 coupling (to 14N) pattern and a i HMBC cross peak between the H-l 1 and isonitrile carbon (160.2 ppm) signals, X had to be the isocyano group and this meant that Y was the chloro group. The HMBC experiment also showed that a quaternary carbon (C-12) bearing methyl and vinyl substituents was between CHeqNC and CHaxCl as 2J and 3J cross peaks were clearly visible between the H-l 1 and C-3/C-10/C-12/C-13/C-15/C-19/C-20 signals and the H-13 and C-12/C-14/C-15/C-19/C-20 signals. The cyclohexane ring was connected to the indole C-3 via C-10 on the basis of HMBC couplings between the H-10 and C-Z/C-3/C-l l/C-12/C-14/C-15 signals. Long-range zig-zag coupling between H-21E and H-13 (0.7 Hz) strongly suggested that C-12 had the S* configuration (same as in 43), not R as in 1.3