This investigation compared several physico-chemical variables important to diatom and protozoan communities and examined the relationships between these variables and communities. Polyurethane foam (PF) substrates were placed at  15 cm depth in five lentic habitats (three lakes, a bog, and a marsh) in northern Michigan during the summer of 1983. Three substrates were removed after 1, 3, 7, 14, and 42 d of field exposure, and presence or absence of protozoan speices was recorded from living material, while diatoms were enumerated from preserved samples. Concurrent with PF substrate removal, morning water samples were collected andanalyzed for pH, temperature, alkalainity, conductivity, dissolved oxygen, chloride, silica, ammonium, and ortho- and total phosphate. Physico-chemical variables examined singly and collectively revealed that the bog and marsh were quite different from each other and the lakes, while the three lakes were similar in physico-chemical composition. Factor analysis revealed three factors that collectively explained 84.99% of the environmental variability. Detrended correspondence analysis (DCA) performed on the biological presence-absence data revealed clusters of diatom samples for each of the lentic habitats. The DCA plots of protozoan communities suggested that bog and marsh samples were different; however, samples from the three lakes were similar. In addition, pH showed the strongest correlation between both community divisions. The DCA sample scores, when correlated with the environmental data, showed that diatom communities had the greatest number of significant correlations with environmental variables. This, coupled with the greater clustering of the diatom samples, implied a greater degree of relationship between diatom communities and the physico-chemical environment.