Generic mixed-flask culture microecosystems derived from small lake and pond planktonic communities were used to evaluate the effects of selected alcohols, aniline derivatives, and aromatic amides on ecosystem-level state variables. The biogeochemical state of the systems was characterized by pH and dissolved oxygen, which reflect energy flow and nutrient cycling processes within the systems. Using changes in these variables as indicators of effect, the relative toxicity rankings of compounds within each group were determined and compared with the toxicity rankings established by fathead minnow acute toxicity. The ecosystem-level relative toxicity of three alcohols followed the toxicity rankings based on fathead minnow 96 hr LOEL's (lowest observed effect levels). The toxicity rankings of aniline and three derivatives were similar to those of the alcohols. In both alcohols and anilines, ecosystem-level effect measures became more sensitive than fathead minnow lethality toward the least toxic member of the group. These examples can be considered representative of situations that might be encountered in an early stage of ecosystem-level testing.