Much of the natural history of the endangered crawling water beetle, Brychius hungerfordi, is unknown. Population estimates have only been surmised. We have measured, using a mark and recapture technique, a localized population of the beetle in a pool on the East Branch of the Maple River, Emmet County, Michigan. The average population during July 2001 was found to be 1052 individuals with a range of 43 (highest estimate–lowest estimate). A three-year study of the relative abundance of the beetle in the same pool suggests that the population has remained relatively constant.