Mercury is one of the most hazardous metals that may contaminate estuarine ecosystems and induce toxic effects on wildlife organisms. It has been suggested that impairment of cholinesterase (ChE) activity may be involved in the resulting mercury toxicity. Following Palaemon serratus exposure to mercury chloride (HgCl2), no effect on ChE activity was observed whatever the concentration used (to 37.5 microM) or the time of exposure (to 7 days). By contrast, following 24 h exposure to dichlorvos, an organophosphate insecticide with a well-characterised anti-ChE action, decrease of ChE activity was observed until 30 to 40% basal activity, which seems to be the minimum activity required for prawn survival. In addition, HgCl2 does not affect dichlorvos toxicity and treatments with a mixture of both compounds can be interpreted as the sum of the two independent toxicities. Therefore, mercury and insecticide toxicities are independent and ChE activity from P. serratus eyes seems to be a reliable and sensitive biomarker for organophosphate insecticides even when organisms are simultaneously exposed to mercury.