Peridinium bipes exerted an inhibitory effect on the growth of Microcystis aeruginosa. The algicidal action of water-soluble extract from P, bipes was studied. After treatment with P. bipes extract, the absorption spectrum of M. aeruginosa culture changed markedly, particularly in the ranges of 500 similar to 650 and 420 similar to 460 nm. An increase in absorption in this wavelength resulted from a leakage of phycobilines, both phycocyanin and allophycocyanin, from the treated cells. However, no leakage of chlorophyll was detected. The leakage of phycobilines was a short-term effect, detectable within 1 h of incubation. Both the plasmalemma and thylakoid membranes in M. aeruginosa cells were damaged and distorted in fine structure after treatment with P. bipes extract. It is assumed that algicide from P. bipes exerts its effect on the cell membranes, giving rise to changes in membrane permeability and a dissociation of phycobiline assemblages, but not of chlorophyll complex, on the thylakoid membranes. As a result, phycobilines were leaked out from the cells.