In 1983 and 1984 I studied the gypsy moth, Lymantria dispar (L.), predator, Calosoma sycophanta L., in a collapsing gypsy moth infestation. Beetles caught in tree-trunk barrier traps were marked and released to assess the directions and distances they dispersed and their population sizes. In both years, about 700 to 1,000 adult beetles of each sex were in the 4-ha site; females were slightly less abundant than males. In 1983, gypsy moths were abundant, male and female beetles were active, and many beetle progeny were produced. Male beetles were more mobile than females and were trapped more often. In 1984 few gypsy moths were present and although the beetle population was substantial, very few progeny were seen and adults dispersed little. Thus, beetle behavior differs substantially at different prey abundances. By examining gypsy moth pupal remains, I found that the progeny of 1983 beetles destroyed three-fourths of the gypsy moth pupae located on tree trunks. C. sycophanta may, thus, have a substantial impact on gypsy moths.