The genetic typing of laboratory strains of Daphnia magna revealed that although there are genetic differences between different testing laboratory strains, most laboratories culture single genotypes as clones. A comparative study was made of the performance of five of the more common clones in terms of neonate mortality under acute exposure to nine toxic chemicals. The range in clonal EC50 response was from less than one to more than two orders of magnitude in terms of concentration. Surprisingly, there was little pattern in rank order of response of clones exposed to different toxicants, thus suggesting that genotype-environment interactions were predominating. Nevertheless, it was suggested that specifying the genotype used in D. magna toxicity tests will result in increased test repeatability. Evidence indicating that culture environment may also significantly affect the performance of neonates in acute toxicity tests was also presented.