The dynamics of thimbleberry aphid [Masonaphis maxima] on its wild perennial host [Rubus parviflorus] are compared at different parts of its geographical range, and the external factors which restrict both the dynamics and the range are examined. The insect occurs along the Pacific coast from British Columbia [Canada] to California [USA]. In any one region different populations have sharply different densities, predation loads and lengths of season, but successive generations are well synchronized in all populations of the region. Population dynamics in California are similar to those further north, but the insects'' temperature requirements are adjusted to local conditions. The edge of the range is determined by the time (3 generations) needed to maintain existing populations in every season, plus the extra time (2 generations) taken to cross gaps of 1 km or more in the distribution of host plants. Populations at the edge of the range are as dense and persistent as those at the center. Egg hatch is closely timed to bud break in the host plant. The relative survival and reproductive rates of early- and late-hatching fundatrices are compared. Fecundity and proportion of sexual males are determined by host plant quality and by non-genetical maternal effects. There is little or no genetical variation. The density-dependent reduction in fecundity may be due to competition between adult mothers and is equally severe in different populations.