An aerial form of Scytonema mirabile (Dillwyn) Bornet, designated strain number BY-8-1, has been found to contain six novel isonitriles, viz. mirabilene isonitriles A-F (7-12), which are mildly cytotoxic and antimicrobial. The gross structures of 7-12 were determined by detailed spectral analysis. The relative and absolute stereochemistry of 7-12 were solved by chemical degradation and direct comparison of degradation products with synthetic samples; mirabilene-A isonitrile (7), for example, was degraded to methyl (3R,R,7S,9S)-3,5,7,9-tetramethoxy-10-oxoundecanal (15) and isopropyl (S)-3-trifluoroacetamidobutyrate (14), which indicated that the absolute configurations of C-4, C-6, C-8, C-10, and C-16 in 7 were all S. Hitherto isonitriles (e.g. hapalindoles) had been found only in blue-green algae belonging to the Stigonemataceae. In this paper we describe the total structures of the mirabilene isonitriles.