Cassida rubiginosa Müller (Coleoptera: Chrysomelidae) has attracted considerable interest as a potential biological control agent against thistle (Astaraceae: Cardueae) weeds, but population characteristics of this species are not well understood. We conducted an intensive mark–recapture study from 1996 to 1999 in Yuwaku, Kanazawa, Japan, to estimate adult population parameters such as population size, longevity, movement activity, reproductive rate, and winter survival. Overwintered adults began to emerge from hibernation in early April and actively moved among host plants until early May. Thereafter, adult dispersal activity decreased until late June, and the number of beetles gradually declined. New adults were active for only 2 to 3 wk after emergence and began aestivation at the end of August. Despite low reproductive rates per generation, the overwintering population was stable at a high density. Beetle longevity was substantial, and many individuals overwintered more than once. The age structure of adult beetles was responsible for the high stability of the population and low reproductive rate.