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Acute and chronic ecotoxic effects of organophosphorous insecticide malathion (Fyfanon 50 EC 500 g L−1) were investigated on three strains ofDaphnia magna. The nominal effective concentrations immobilizing 50% (EC50) ofDaphniaafter 24 and 48 h were 0.53 and 0.36 μg L−1, 0.70 and 0.44 μg L−1, and 0.75 and 0.46 μg L−1for the strains 1, 2, and 3 respectively. There was an increase in malathion ecotoxicity with time of exposure as confirmed by chronic studies. In fact, after 21 days of exposure, significant effects on survival (lowest observed effect concentration or LOECs) were recorded at malathion concentrations of 22, 220 and 230 ng L−1for strains 1, 2 and 3 respectively. Other endpoints were also examined, including reproduction (with different parameters), body length and embryo toxicity. ICs10 and ICs20 were calculated for these different parameters. ICs10 ranged from 4.7 to more than 220 ng L−1for the three tested strains. The most sensitive parameter was the number of neonates per adult (ICs10 = 4.7 and 10.8 ng L−1for strains 1 and 2 respectively) or the number of broods (IC10 = 10 ng L−1for strain 3). Moreover, an increase in embryo development abnormalities was observed at the two highest tested malathion concentrations. Abnormalities comprised undeveloped second antennae, curved or unextended shell spines, and curved post-abdomen spines in live neonates. Results suggest that malathion could act as an endocrine disruptor inD. magnaas it interferes with development. It also induces a significant decrease in acetylcholinesterase (AChE) activities for the three strains. Both strains 2 and 3 seemed more sensitive (LOECs = 60 ng L−1) than strain 1 (LOEC = 120 ng L−1). Our results suggest that the AChE activity response can also be used as a biomarker of inter-strain (or inter-clonal) susceptibility (i.e. strain (or clone)-specific response). [ABSTRACT FROM PUBLISHER]