This thesis examines the toxic effects of ethyl parathion and polluted seawater on the polychaete worm Ophryotrocha diadema. Short and long term experiments were conducted to study lethal and sublethal impacts on mortality, growth, and reproduction. The larval development of O. diadema was also described. Results showed larvae were more susceptible than adults to ethyl parathion. Growth was significantly reduced at concentrations of 0.9 ug/L. The thesis also discovered the egg-eating behavior of O. diadema, which may influence interpretation of previous toxicity tests using this species.