Slime-covered rocks and samples of process waters from two trickling filters for treatment of municipal wastes were brought to the laboratory for probing with microelectrodes to determine dissolved oxygen (DO). Slime thickness was 0.4–1.5 mm. Flow rate of medium over the slime had a minor effect on slime respiration, but pH 5 or below was strongly inhibitory. Increasing temperature showed lower oxygen concentration throughout a slime, although 27°C had results little different from those at 22°C. Medium concentration had a profound effect on oxygen concentration profiles, and either oxygen-limited or substrate limited respiration could be demonstrated. Illumination of slimes from the top of the trickling filter developed oxygen supersaturation because oxygen from photosynthesis could not diffuse away rapidly.