We evaluated the effect of 3 sublethal concentrations of Cd (as CdCl(2), 209.50, 104.75 and 52.38 microg/L) and Zn (as ZnCl(2), 283.88, 141.94 and 70.97 microg/L) separately and together on the population growth of the common Mexican brachionid rotifer Brachionus havanaensis using 1 x 10(6) cells/mL of Chlorella vulgaris as the algal diet. The LC(50) (24-h bioassay conducted in the absence of food) for B. havanaensis was 419.0 +/- 8.1 microg/L for Cd and 2271.0 +/- 404.4 microg/L for Zn. The effect of different concentrations of Cd or Zn on the population growth of B. havanaensis showed decreased population abundance with increasing levels of the metal concentration in the medium. However, for Cd even at the highest tested concentration (i.e., 50% LC(50), 209.5 microg/L), there was some increase in the abundance of B. havanaensis, although much lower than in controls. On the other hand, for Zn at highest tested concentration (i.e., 12.5% LC(50), 283.88 microg/L), the population of B. havanaensis showed little growth and remained close to the initial density. Under the conditions of mixed metal exposure, B. havanaensis did not grow at the highest tested concentrations of Cd+Zn combination. The rate of population increase (r) varied from 0.11 to 0.43 per day, depending on the heavy metal concentration. Statistically r was adversely influenced by the heavy metal concentration in the medium. Tests of multiple comparisons indicated that r at the lowest level of Cd or Zn (alone or mixed) was not significantly different from controls. Our data from the acute toxicity tests showed that Cd was 5 times more toxic than Zn to B. havanaensis but this difference was much reduced based on the chronic toxicity studies.