The cyanobacterium, Fischerella muscicola, produces a secondary metabolite named fischerellin A (FS) that strongly inhibits the growth of cyanobacteria and other photosynthetic organisms. The compound exhibits a unique structure and is composed of two cyclic amines and a C₁₅ substituent that contains a double bond in the (Z) configuration and two triple bonds [L. Hagmann, F. Jüttner, Tetrahedron Lett., 37 (1996) 6539-6542]. The site of FS action is located in photosystem II (PSII). The chlorophyll fluorescence induction transient and O₂ evolution methods have been used to determine the site of action of FS in PSII. FS affects the fluorescence transients, as well as O₂ evolution by the cyanobacterium, Anabaena P9. The green alga, Chlamydomonas reinhardtii, and higher plants were also affected by FS in a concentration- and time-dependent fashion. FS acts at several sites which appear with increasing half-time of interaction in the following sequence: (1) effect on the rate constant of Q_A^- reoxidation; (2) primary photochemistry trapping; (3) inactivation of PSII reaction center; and (4) segregation of individual units from grouped units. FS does not affect the photosynthetic activity of purple bacteria, Rhodospirillum rubrum.