Yeast extract, a component of microbial culture media, contained compounds toxic to species of Microcystis (cyanobacteria, blue-green algae). The compounds were identified as lysine and malonic acid. Cells of Microcystis viridis (A. Brown) Lemmermann (105 cells ml−1) were completely killed within 48 h by l-Iysine at a concentration of 1.0 μg m1−1 (1.0 ppm), whereas lysine was non-toxic to Anabaena species at the same concentration. Also, cells of M. viridis were completely killed by malonic acid at a concentration of 40 μg ml−1 (40 ppm).