The gypsy moth (Lymantria dispar L.) is one of the most abundant and hazardous forest pests. This pest is chiefly destructive to foliage of oaks. At times, infestations of the gypsy moth are discovered in different areas in Belarus (Marchenko, 1998). Numerous studies (Vasechko, 1990; Logoida, 1992; Liamtsev, 1997) were conducted on population characteristics of this pest. Most of investigators describe the population when mass outbreaks occur (Benkevich, 1984; Nealis and Erb, 1993). However, of great interest is the study of the gypsy moth population when its abundance is low since it is important to define factors that govern its development. We did research on the gypsy moth population at a stationary plot of 15.5 hectares that can be regarded as a reservation of a chronic infestation of this pest in the Gomel Province. This is a 50-year-old insular stand of Yield Class V surrounded by ameliorated agricultural land. The percentage of oaks is 86.4%. The stand density is 0.6-0.7. The environmental conditions (loamy sand and deep groundwater) do not favor the development of oak trees there. Perhaps, these unfavorable conditions are responsible for physiological deterioration of the stand which is thus has become a reservation of the gypsy moth.