The effects of 17 toxicants on the growth of five species of algae in pure culture were studied. The two species displaying the greatest sensitivity to the action of each of the compounds tested were Monochrysis lutheri and Phaeodactylum tricornutum , and the most resistant species was Protococcus . Of eight different classes of toxicants tested, substituted urea compounds and a mercuric compound were most effective in inhibiting growth of all algal species at the lowest concentrations.