Mirabimide A, the major imide from the terrestrial blue-green alga Scytonema mirabile (Dillwyn) Bornet (strain BY-&l), is N-(2-(N,N-dimethyleucyl)oxy-3-methylpentanoyl-N-methylvalyl-prolyl]-4-methoxy-5-isopropyl-3-pyrrolin-2-one. Mirabimides B-D are the valyl, 2-oxy-3-methylbutanoyl, and N-acetyl-N-methylleucyl analogs, respectively. The absolute configurations of the five asymmetric carbons in the mirabimides are proposed to be all S, but the assignments for the dimethylleucine units in A-C and the hydroxyacid units in B-D are tentative. Naturally-occurring N-acylpyrrolinones are associated with sponges of the genus Dysidea, the marine blue-green alga Lyngbya majuscula, and gliding bacteria. We report here the isolation and structure determination of four new cytotoxic N-acylpyrrolinones, mirabimides A-D (1 - 4), from the terrestrial blue-green alga Scytonema mirabile (Dillwyn) Bornet (strain BY-&l).