This is an extension of earlier studies of the effects of individual differences on population dynamics. The test species is the western tent caterpillar, Malacosoma pluviale (Dyar), which exhibited qualitative changes in its collapsing population on southern Vancouver Island between 1956 and 1959.There are preliminary descriptions of recently established categories of individuals differing in total activity and behavior, and of the biological consequences of mixing different proportions of these active and sluggish larvae in colonies, A colony with enough active individuals built several long, clavate tents, dispersed them widely, and foraged far away from them. A colony with many sluggish individuals seldom constructed more than one compact, pyramidal tent, and fed nearby. Sluggish colonies were less viable than active colonies in harsh environments, and some were too sluggish to survive in the most favorable circumstances.Between 1956 and 1959, fourth-instar colonies along 156 miles of roadways in the outbreak area decreased from 74,000 to 251. In any infestation, the proportion of active colonies declined from nearly 100% to 45–55% as years of residence in the locality increased from 1 to 4. Thereafter, the proportion of active colonies rose again as those too sluggish to survive began to disappear from the population. The initial percentage decline in active colonies occurred whether abundance was increasing or decreasing. The later percentage increase began only while numbers were decreasing.As infestations aged, even active colonies decreased in size and activity, but the year of minimal density was accompanied by increases in colony size and vitality. This rejuvenation also was reflected in data from adults and egg masses. Methods of using egg-mass data to determine the proportions of different kinds of colonies in infestations of different ages are discussed, and adult activity indices based on scale loss and wing tearing are presented. Examples from all sources indicated declining vitality while the population aged, followed by sudden recovery at or near minimal abundance when its least viable portion had been eliminated.