An artificial fresh-water protozoan community was subjected to different concentrations of Zn and Cu in a test system consisting of Plexiglas troughs through which pond water flowed continuously. Although the % survival of colonizing species exposed to Cu or Zn fluctuated greatly at each concentration, the range of toxicity for each compound allowed comparison of protozoa with other organisms with respect to resistance to heavy metal toxicity. Individual protozoan species also were exposed for 3 hr to Zn, Cu, Cr, phenol, Pb, Mn, Co, HNO3, acetic acid, Al, Sn, and HCl to derive time to death curves. Protozoa tested appeared to be more resistant than Daphnia to phenol, K2Cr2O7, and Cu; however, some species were more sensitive than Daphnia to Zn, nitric acid, and HCl. This suggests that the sensitivity of protozoa to toxicants may be either more or less than that of macroinvertebrates and that information does not suffice to predict sensitivity. Moreover, the relative sensitivity of protozoa to various toxicants will not always be the same, i.e. species X may be twice as tolerant to a toxicant as species Y but its relative sensitivity may be quite different for another toxicant.