A chronic toxicity test using population growth of an aquatic oligochaete, Aeolosoma headleyi, was evaluated for usefulness in determining the hazard of chronic exposures to cadmium. Tests were conducted in artificial hard water (180 mg/L hardness) and dechlorinated tap water (60 mg/L hardness). After 10 to 14 d of exposure, there were significant differences between groups in population growth rate or number of worms in both water types. The no-observable-effects concentrations (NOEC) for replicate tests in artificial hard water were 32.0 and 53.6 micrograms/L cadmium. The 48-h LC50 in artificial hard water was 4,980 micrograms/L, yielding acute/chronic ratios of 155 and 92, respectively. The oligochaete was more sensitive in the softer dechlorinated tap water, with a NOEC of 17.2 micrograms/L cadmium and a 48-h LC50 of 1,200 micrograms/L. The acute/chronic ratio was 70. Dose-response curves were linear at higher concentrations, but stimulation of reproduction was noted at low concentrations.