Changes in the food reserves and chemical constituents of a population of Dugesia tigrina (Girard, 1850) from Grenadier Pond, Toronto, Ont., have been followed over a 13-month period from October 1968 through October 1969. The parameters examined were individual worm dry weight, protein nitrogen, nonprotein nitrogen, lipid nitrogen, lyo-glycogen, desmo-glycogen, and total lipid.Whereas the nitrogen, lipid, and desmo-glycogen fractions remained relatively constant when expressed on a weight-to-unit-weight basis, irrespective of changes in the number and size of individual worms, the lyo-glycogen fraction decreased exponentially with an increase in the number of worms in the population. This decrease mirrored the relation between the size of individual worms and the number of worms in the population. The negative regression coefficient of lyo-glycogen on population biomass was statistically significant (P < 0.05).The changes at different times of the year in the chemical constituents and food reserves of D. tigrina have been correlated with variations resulting from the life cycle of D. tigrina in both the size of the population and the size of the individual members comprising the population.The results are consistent with Reynoldson's (1966) hypothesis that triclad populations show restricted fluctuations in numbers because the population size is regulated by intra-specific competition for a limited food source. The significance of a limited food resource is demonstrated by showing that, as the population size increased, intraspecific competition resulted in decreases in the lyo-glycogen food reserves at both the individual and population level.