Results from single-species toxicity tests form the basis for guidance about concentrations of chemicals that are likely to be environmentally hazardous. Microcosm bioassays have been proposed as better tools for this purpose under some circumstances. Rarely have comparisons been made among the responses in the various test systems and those seen in natural communities. The authors compared the responses among single-species algal assays, a synthetic microcosm (the Taub microcosm) and experimental ponds exposed to similar concentrations of atrazine by comparing concentrations of atrazine that reduced algal activity or biomass to 50% of control values(EC50). Eight algal species were examined; mean species EC50 values were 103 to 159ug/L for 14C uptake ranged from 37 to 308 ug/L. For the Taub microcosm, EC50 values were 103 to 159 ug/L for 14C uptake, 126 to 165 ug/L for dissolved oxygen production and 106 to 164 ug/L for dissolved oxygen consumption. Values were dependent on the time intervals selected for calculations. In the ponds, EC50 values for 14C uptake were 100 ug/L and for chlorophyll a, 82 ug/L.