The development of community level toxicity testing protocols would allow for the direct determination of toxicant effects on community level processes. Using protozoan communities colonized on artificial substrates for testing the effects of hazardous substances allows rapid testing in a minimal amount of space. These communities were used in (1) 48-h acute tests, (2) a 28-day static test, and (3) 10-day continuous flow tests to assess the usefulness of structural and functional indices in determining adverse effects of the lamprey larvicide 3-trifluoromethyl-4-nitrophenol (TFM). Acute tests used protozoan communities of differing ages and were relatively insensitive to the toxicant; the EC50 was 48.0 ppm. Static tests used protozoan colonization as an index of stress and showed a significant increase in Seq and a significant decrease in G at low (0.1 ppm) TFM levels. A significant reduction occurred in ciliate species at 10 ppm. Continuous flow tests had high variability in colonization within treatments, and no significant concentration differences were detected. Functional group analyses showed no effect on community function, although higher trophic levels were reduced or absent at 10 ppm. Results support the use of multispecies tests in determining toxic effects on natural communities, while showing the need for continued work in this area to develop standardized, replicable testing procedures.