SUMMARY (1) A population of the erpobdellid leech Erpobdella octoculata (L.) was studied over a period of 16 months in the organically polluted River Ely, South Wales. The life cycle was completed in one year. (2) Seasonal patterns of mortality were apparently related to flow, temperature and life-cycle stage. Three distinct periods of exponential growth were observed during the life cycle and the growth pattern was consistent from year-class to year-class. Annual population production was 29.4 g wet weight m-2 y-, annual population biomass was 5.6 g wet weight m-2 and the annual P/B ratio was 5.3. (3) A detailed comparison between different populations of E. octoculata revealed considerable variation of life cycles, growth, development and reproductive biology, the basis of which is not yet established.