The taxonomy of L. udekemianus is discussed briefly. 2. The development of the reproductive organs and the life history of this species under labo- ratory conditions are described. 3. The life history and seasonal changes in num- bers of L. udekemianus have been followed in three localities: a productive river and two unproductive streams. 4. This species has a straightforward biannual cycle. Breeding takes place in late winter or early spring and is followed by death of the worms and production of cocoons. The young take at least a year to attain maturity, and breed in their second year of life, and the population at any one time contains a mixture of two year-classes. 5. It is suggested that differences between the time and duration of breeding in different localities are related to their productivity. 6. Seasonal changes in population size may be related to breeding activity, climatic condi- tions and intensity of predation. Pe3ioMe