The life history, fecundity, behavior and feeding habits of the North American leech, Erpobdella punctata, are investigated. There is evidence of an upstream migration in early spring. The size distribution within populations indicates that growth to maturity took one year in a permanent pond, but two years in a stream, perhaps because the stream dried up in summer. It seemed that few survived to a second breeding season. Mortality in the stream was estimated at about 93%, 73% and 93% in the first, second and third years, respectively. Intraspecific and snail predations of cocoons were important causes of mortality. These leeches are scavengers and predators, rather than parasites. Courtship involved mutual stimulation and cocoons were laid primarily in May, correlating with a large increase in water temperature. Each individual laid approximately 10 cocoons, each with five eggs which hatch about 3-4 weeks after being laid. INTRODUCTION Erpobdella punctata (Leidy, 1870) is one of the most commonly encountered and widely distributed species of freshwater leeches in North America. Known since 1870, it remains the only substantiated American representative of the Holarctic genus Erpobdella de Blainville 1818 and occurs in Alaska, Canada, and most of the United States to Mexico. Apart from brief scattered references, little is known about such aspects of its biology as its breeding habits, fecundity, population structure or ecology. By contrast, close European and Asian relatives of E. punctata have been studied extensively by such workers as Bennike (1943), Brandes (1899), Brumpt (1899, 1900), Jijima (1882), Klekowska (1951), Luferov (1963), Mann (1953, 1955, 1956, 1958, 1961), Nagao (1957), Pawlowski (1955), Sandner (1951) and Soos (1966). The biological observations on E. punctata presented in this paper constitute an attempt to fill this void, serving for the comparison of E. punctata with its related forms. METHODS OF STUDY The intent of the study was to make detailed biological observations on small natural populations of the free-living erpobdellid, Erpobdella punctata. Two especially suitable study sites in the vicinity of Ann Arbor, Michigan, were chosen on the basis of the abundance of E. punctata and the ease with which they could be observed. One site was a small stream in Hudson Mills Park on the Huron River, approximately 2 mi NW of Dexter, and the other Earhardt Pond, located about 0.5 mi N of the Concordia Junior College, just outside