Daphnia magna were exposed to cadmium, copper and cyanazine to determine the relative sensitivities of several age groups: less than 4 h, less than 24 h, 1 d, 2 d, 3 d, 4 d, 5 d and 6 d old. Mean cadmium 48-h EC50 values for each age group ranged from 23 to 164 micrograms/L. Mean copper EC50 values ranged from 6 to 18 micrograms/L. Cyanazine EC50 values ranged from 53 to 106 micrograms/L. The 1-d-old Daphnia mean EC50s were 48 and 49 micrograms/L for cadmium, 10 and 10 micrograms/L for copper and 84 and 86 microgram/L for cyanazine, respectively. These similar sensitivities indicate that older animals can be used in tests equally as well as younger animals, thus simplifying the recovery of daphnids in acute sediment toxicity tests.