This paper is a report of bacteriological analyses made on bottom deposits of Crystal Lake, an oligotrophic lake in the Northern Wisconsin State Forest. Twenty-nine mud samples representing eight cores were studied. Anaerobic bacteria as well as aerobic and facultative bacteria decreased in numbers at increasing depths in the mud. Bacterial counts are much lower than similar reported counts for bottom deposits of the eutrophic Lake Mendota, in Southern Wisconsin. This is correlated with the lower content of organic and mineral matter in Crystal Lake water.