This study was carried out to investigate the acute toxicity of endosulfan, an organochlorine pesticide, and the effect of a sub-lethal concentration of this compound on the rate of oxygen consumption (Mo2) of three different Spanish strains of Artemia nauplii. The 24h LC50 values showed that the nauplii of the parthenogenetic diploid strain were more resistant, whereas those of the parthenogenetic tetraploid and bisexual strains were more sensitive to endosulfan anddid not show differences between them. The results suggest that the use of different Artemia strains, with their differing degrees of sensitivity to the same toxicant, may be a valuable tool in aquatic ecotoxicological research. Exposure to sub-lethal concentrations (1/5-24hLC50) of endosulfan had no significant effect on the oxygen consumption rates (Mo2) of each strain of the nauplii. Nevertheless, a reduction in the ability of the nauplii to maintain respiratory independence during hypoxia, after 24 h exposure to such dose, was observed. [ABSTRACT FROM AUTHOR]