Density of the red pine scale,Matsucoccus resinosaeB and G (Homoptera: Margarodidae), and the related resinosis of its host,Pinus resinosaAit., in a Connecticut plantation had a profound influence on scale biology, behavior, and population dynamics. Fecundity of the fall, 1975 generation was positively correlated with the density of feeding nymphs on 9 trees. During development of the subsequent summer, 1976 generation, the most heavily infested trees began to secrete resins from feeding wounds. Nymphs feeding on these trees required 2–3 wk longer to mature and females produced significantly fewer eggs than their mothers.Overwintering mortality was greatest among nymphs in the early substages of the 1st instar and among nymphs which colonized 1- and 2-yr-old growth. Since nymphal development was prolonged on heavily infested trees, a proportionately greater number of 1st-instar nymphs overwintered in earlier substages of development. Also, resinosis at the preferred colonization sites (3-yr-old growth) on these heavily infested trees forced crawlers to settle on the less desirable young growth. Consequently, nymphs on heavily infested trees suffered a greater percent overwintering mortality than did nymphs on less heavily infested trees.Dispersal ofM. resinosaecrawlers was density-dependent.