Puwainaphycins A-E, one of which (C) is a potent cardioactive agent in isolated mouse atria, are cyclic decapeptides that have been isolated from a terrestrial blue-green alga Anabaena sp. BQ- 16-1. The structures of puwainaphycins A-E, including most of the stereochemical features, have been elucidated by a combination of spectral and chemical methods.