In the present work, we quantified the harmful effects of Cd(+ 2) to Brachionus macracanthus using both acute (median lethal) and chronic (population growth) toxicity tests. Chronic toxicity tests were conducted under 4 different concentrations (0.000625-0.005 mg L(- 1)) of cadmium chloride at 23 degrees C under 3 food (Chlorella vulgaris) levels (0.5 x 10(6), 1.0 x 10(6) and 2.0 x 10(6) cells mL(- 1)) using static renewal system for three weeks. The median lethal concentration bioassayed at 24 h (LC(50)) for B. macracanthus was 0.19 mg L(- 1) of CdCl(2). Cadmium adversely affected the population growth of B. macracanthus at all tested concentrations. Increase in algal food had a positive effect on the rotifer growth in controls; with increase in Cd levels, the population growth of B. macracanthus decreased even under the highest food level tested. The peak population abundance of B. macracanthus in controls at the highest food level of 2.0 x 10(6) cells mL(- 1) was 40 ind. mL(- 1). Depending on the heavy metal concentration and the algal level, the population growth rate (r) of B. macracanthus varied from 0.02 to 0.28 day(- 1). The relatively higher sensitivity of B. macracanthus to cadmium toxicity is discussed in relation to other species of the same genus.