Alkylphenols are widely distributed in the marine environment and we have found them to be widely dispersed in New England lobster populations. In vertebrates, these compounds act as estrogenic endocrine disruptors. Here, we tested two alkylphenols, bisphenol A and 2,4-bis-(dimethylbenzyl)phenol for their effects on lobster larvae. In particular, we examined their toxicity and their effects on molting and metamorphosis. Lobsters were raised to stage 2 and were then isolated individually in ice cube trays with nylon mesh netting on the bottom giving access to flow through cooled seawater. The experimental larvae were fed with 5 or 10 ng alkylphenol per day in their diet. Controls were fed the same diet minus the test chemicals. The alkylphenols were toxic to second and third stage larvae, and fourth stage post-larvae, with 13-21% surviving to 25 days compared to 69% of controls. The compounds significantly delayed each molt by 2-3.5 days, and the surviving treated larvae were predominantly larval juvenile intermediates at metamorphosis (62%). The differences between treatments and controls in all cases were highly significant using survival analysis based on the Weibull distribution. We conclude that alkylphenols are endocrine disruptors to lobster larvae at metamorphosis because they possess juvenile hormone activity. They also delay molting, reduce growth, and are toxic at relatively low concentrations.