Two species of leeches, Erpobdeila octoculata (L.) and Glossiphonia complanata (L.), were found in the Wilfin Beck, a small stony stream in the English Lake District. The population of G. complanata was very small and no detailed study of this species could be made. The present paper describes the life cycle of Erpobdella octoculata, and provides estimates of mortality rates, growth rates, standing crop and production. A second paper (Elliott 1973) describes the diel activity pattern, downstream drifting, and food of the leeches. This species is almost always the most numerous leech present in running water (Mann 1964), and was abundant in some sections of the Wilfin Beck. Mann (1953) studied a population in a Berkshire stream from April to September 1952, and described a life cycle which was more complex than that found in the Wilfin Beck. Therefore some useful comparisons can be made between the two studies.