The free-living nematode, Caenorhabditis elegans (C. elegans) was adopted as a multicellular biosensor of biological toxicity from alkylphenols and organotin compounds. Alkylphenols were found to affect reproduction at lower doses than indicated by the acute toxicity assay. In particular, nonylphenol altered the reproduction rate of C. elegans at a dose 10-to 100-fold lower than the 50% lethal concentration (LC 50 ). A comparison of the number of viable worms and eggs suggested that alkylphenols and organotin compounds possess hatching toxicity. A 0.1 µM dose of organotin compounds caused a significant decrease, in the order of 20-50%, in reproduction of the worms, and an abnormal male: hermaphrodite ratio was observed. C. elegans therefore appears to represent a potent and sensitive organism with which to evaluate the biological effects of chemicals. In particular, the sensitivity of reproduction as an endpoint is highly useful for assessing the sublethal effects of chemicals.