The behavior of lead and cadmium in a protozoan community was studied in order to obtain new data regarding the toxicity and bioaccumulation of these heavy metals. For this purpose, microcosms with different concentrations of the pollutants (without metals, 500 microg Cd and Pb x L(-1) 1,000 microg Cd and Pb x L(-1)) were used. Protozoans bioaccumulated 7.03-207.00 microg Pb x g(-1) dry weight (dw) and 0.05-332.75 microg Cd x g(-1) dw, representing an accumulation capacity of up to 161.45 microg Pb x g(-1) dw and 310.75 microg Cd x g(-1) dw more than that of the bacteria. The addition of both metals caused a significant reduction in the density of protozoans. These data were compared with those obtained previously in treatments that used these metals separately.