The relative toxicity of certain short chain fatty acids, dicarboxylic acids, alcohols, aldehydes and some selected organic compounds to Chlorella pyrenoidosa were evaluated. Changes in chlorophyll content of the algal suspension were measured to determine toxicity. In addition, studies were conducted to investigate the effect of certain inactivators in reducing the toxicity of some of the organic chemicals. The results show that modifications to industrial waste treatment plant design practices are needed, since decreases in stream reaeration rates may be significant if certain chemical wastes are discharged. A general relationship exists between toxicity and molecular structure of the chemicals investigated.