Abstract The acute toxicity of zinc and lead in water with clean sediment to the polychaete Hediste diversicolor was evaulated by static 10-day and 28-day bioassays, calculating the LC50 (lethality concentration for 50%). Mortality has increased with increasing concentrasions of zinc and lead. The results indicated that Zn was more toxic to the species than Pb. The results also showed that small worms are more sensitive to Zn and Pb than bigger worms.