Fecundity and colony size of Hyphantria cunea Drury were measured in six census areas for 13 years, and wing length was measured in one area. Colony size in the fifth instar was closely correlated with fecundity, and the percentage mortality from the egg stage to the fifth instar was an inverse function of fecundity. Wing length was not a sensitive index of fecundity where there was insufficient competition for food to cause severe reduction in moth size.Seasonal differences in fecundity and colony size within years, as well as differences between areas and between years, followed patterns expected from laboratory studies. Decreases during each season, proximately attributable to the age of foliage available to the larvae and ultimately attributable to genetic differences in the heat requirements for diapause termination, made it difficult to estimate mean fecundity for any area or year. Fecundity seemed slightly higher in coastal than in inland areas. Both population quality and weather appeared to influence annual differences, but because of the complex interaction between them and the high variability in fecundity and colony size, the relationships could not be modeled effectively with the available data.Differences in fecundity and colony size associated with both time and place were negligible in comparison to the changes in the population density of colonies, as measured in the same areas over the same period of years. It is concluded that priority should be given to survival rather than fecundity in the development of population models for H. cunea, at least in areas where competition for food is not severe.