Various algae species were tested for their susceptibilities towards chlorinated hydrocarbon insecticides. Deildrin, which is the most frequently found pesticidal contaminant in the US, and its analogs were found to inhibit the growth of certain of algae species. Anacystis nidulans in particular showed marked susceptibility to endrin, dieldrin, ketoendrin and photodieldrin. This species was also susceptible towards dieldrin metabolites such as metabolite F and G. Among DDT metabolites DDD (TDE) was found to be the most toxic material, followed by DDE, DDT and FW-152. It has not been known that DDT should be more toxic to algae. In terms of acute toxicity phenylmercuric acetate was by far the most algicidal agent among all pesticidal chemicals tested. This pesticide is toxic to both A. nidulans and A. quadruplicatum at the concentration of 1 ppb.