Life history of G. complanata was investigated by weighing individually all leeches in samples taken from a tributary of River Pang, Berkshire. Assuming that the population is stable, eggs were laid in 2 distinct broods, 2-year-olds laying in March and 1-year-olds in April-May. Parents each produced an average of 26 young, over which they brooded for about 20 days. At 1 year of age 100% of the early brood were breeding, but only 40% of the late brood. At 2 years of age all were breeding, and most died soon after. A few survived to breed at 3 years of age (probably those which did not breed at 1 year), and an occasional specimen may have been 4 years old. Mortality was 97% of the age-group up to the first October of life, 33% in the next 12 months, and 84-88% in the following 12 months. Only 5-6% of the total populations were 3 years old or more. Early brood and late brood had differing mean weights throughout life. Rates of growth were comparable except that the late brood grew more slowly in 1st few months of life, but faster during the 1st breeding season when many of them were immature. Some evidence that breeding begins earlier in those parts of stream warmed by the sun, and in more dense populations. Outline of life history is similar to that of Erpob-della octoculata.