A compound which would effectively and economically control the growth of nuisance species of blue-green algae with a minimum impact on desirable forms of life in the aquatic environment was searched for. A computerized structure search of more than 100,000 compounds was made to select the analogs of the following four Phase I prime candidates: 2,5-Dichloro-3,4-dinithrothiophene; (5-Chloro-2-(p-nitrophenoxy)phenyl)phenyliodoniumchloride; 4-Amino-2,5-dibromophenylthiocyanate; and 1,1-Dimethyltetradecylamine, hydro-chloride. Through this endeavor 1309 compounds were selected, 41 of which emerged from a rapid, agar-plate screening as candidates for final laboratory screening tests. A golden-brown flagellate, Ochromonas ovalis, which exhibited phagocytic activity against the blue-green alga, Microcystis aeruginosa, was discovered during Phase I. Further research and development of biological-chemical control system included studies involving several species of Ochromonas and conditions which would enhance their phagocytic activity against Microcystis aeruginosa, with Ochromonas ovalis proving to be the most voracious feeder.