Eighteen new indole alkaloids, hapalindoles C-Q and T-V, have been isolated from the cultured cyanophyte Hapalosiphon fontinalis. The gross structures and relative stereochemistries of these 18 minor hapalindoles have been determined by using spectral methods. The relative stereochemistries of hapalindoles A, D, and K have been confirmed and the absolute configuration of hapalindole K has been established as llR,12R,13R by X-ray crystallography. Assuming that all of the alkaloids have the same absolute stereochemistry as K, all of the hapalindoles isolated to date are HR and all of the chlorine-containing ones are 13R. With the exception of hapalindole L, all of the hapalindoles are 12R. Hapalindoles A and B, the major alkaloids in the blue-green alga, and several of the minor hapalindoles are 10R,15S. Some of the alkaloids, however, viz. C-G and U, are 10S,15S and two, viz. H and Q, are 10R,15R. Most of the hapalindoles are tetracyclic, but C-F and Q are tricyclic; none of the tricyclic hapalindoles isolated to date, however, are 10R,15S. Hapalindole T is an unusual thiocarbamate and hapalindole V is 10-hydroxyhapalindole G. © 1987, American Chemical Society. All rights reserved.