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,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. The major cytotoxic, fungicidal agent in the cultured terrestrial blue-green alga Scytonema mirabile (isolate BY-8-1) is tolytoxin (1). Polymethoxyalkenes 2-3, which are related to 4-6 from another tolytoxin-producing cyanophyte Tolypothrix conglutinata var. colorata, are also present in S. mirabile. Upon examining the extract of S. mirabile, another member of the Scytonemataceae, for other cytotoxins and fungicides, a new group of isonitriles was found, viz. the mirabilene isonitriles 7-12, which are mildly cytotoxic and antimicrobial and obviously related to compounds 2 and 3. 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.