The body size differences between neonates and adults of many cladoceran species are large and this influences their susceptibility to toxicants. In this study we quantified the mortality rates based on acute toxicity tests using 7 species of Cladocera (Alona rectangula,Daphnia laevis, D. pulex, D. similis, Ceriodaphnia dubia, Moina macrocopa and Macrothrix triserialis) subjected to stress from methyl parathion (a pesticide) and mercury (a heavy metal). Among the test species, the body size of D. pulex was the largest while that of A. rectangula was the smallest, for both adults and neonates. Regardless of the toxicant, for a given species, neonates were more sensitive than the adults. For mercury, the 24h LC(50) for neonates varied from 5.6 to 16.6 microg/L while for the adults the range was higher (7.6 to 42.5 microg/L); the ratio of LC(50) of neonates to adults of D. laevis was > 3.5, while it was lowest (1.3) for M. triserialis. For methyl parathion, the range of LC(50) for neonates was from 2.5 to 15.6 microg/L, while for the adults it was from 8.2 to 25.9 microg/L. The ratio of LC(50) of neonate to adults was lowest (about 1.1) for C. dubia but was highest (> 5.5) for Alona rectangula. Our study also showed the need to employ a range of cladoceran species for ecotoxicological tests since no single species was consistently sensitive for stress from heavy metal or pesticide.