As part of a study to evaluate laboratory toxicity screening tests that include single species bioassays and micrcosms, the authors examined community level responses in Taub microcosms exposed to atrazine (60,100,200,500,1000, and 5000 micrograms/L), a commonly used herbicide. Measurements of community metabolism included primary productivity, community respiration, primary production efficiency, & productivity/respiration (Pn/Rni) ratios. These community measurements varied in their sensitivity to atrazine. Primary production efficiency (primary productivity per unit chlorophyll) appeared to be the most sensitive measurement with greatly reduced efficiencies occuring throughout the experiment at atrazine exposures of 60 micrograms/L. The other community measures appeared to be more sensitive to atrazine during the interval when Daphnia magna populations were highest, suggesting increased pressure by D. magna on primary producers increased the sensitivity of the test system to atrazine. All community metabolism measurements of the microcosms exposed to higher atrazine concentrations of 500, 1000, and 5000 micrograms/L differed from controls throughout the experiment.