Potamothrix hammoniensis (Michaelsen) (Tubificidae, Oligochaeta) forms a monoculture in the deeper profundal of Lake Esrom. In shallower profundal Psammoryctes barbatus Grube comprises about 5 % of the tubificid population. Life cycle and population dynamics of P. hammoniensis are influenced by environmental and competitive factors. Density of mature specimens was greatest in the deep profundal. Variable length of stratification period (low oxygen content) causes changes in growth, predation pressure and fluctuations in population size. Early autumn overturn in successive years resulted in lowered density due to increased predation. Predators are fish and carnivorous invertebrates. Anaerobic conditions reduced breeding activity and fecundity. High densities of Chironomus anthracinus (Zett.) decreased growth and fecundity of P. hammoniensis and delayed breeding until the worms were four years old. Low densities of C. anthracinus resulted in high densities of P. hammoniensis which bred when three years old. At shallower depths growth is faster, fecundity greater, the average new generation larger, greater mortality is compensated by higher fecundity and shorter life cycle. The shallow water population seems to be controlled mainly by predation and the deep water population by competition.