Rhodococcus sp. strain p52, a previously isolated dibenzofuran degrader, could effectively inhibit the growth of cyanobacteria, including species of Microcystis, Anabaena, and Nodularia. When strain p52 was inoculated at the concentration of $7.7{ imes}10^7$ CFU/ml, 93.5% of exponentially growing Microcystis aeruginosa ($7.3{ imes}10^6$ cells/ml initially) was inhibited after 4 day. The threshold concentration for its algicidal activity against M. aeruginosa was $7.7{ imes}10^6$ CFU/ml. Strain p52 exerted algicidal effect by synthesizing extracellular substances, which were identified as trans-3-indoleacrylic acid, DL-pipecolic acid, and L-pyroglutamic acid. The effective concentrations of trans-3-indoleacrylic acid and DL-pipecolic acid against M. aeruginosa were tested to be 0.5 mg/l and 5 mg/l, respectively.