(1) The population dynamics of four natural populations of Panax quinquefolium were observed in southern Quebec, where the species reaches its northern limit of distribution in North America. P. quinquefolium is a rare forest perennial herb threatened by harvesting and habitat destruction. (2) Prior to demographic analysis by transition matrix models, a choice was made between age or size, considering fecundity and survival variables, for the state classification. Size, expressed in terms of number of leaves, was the most reliable state variable. (3) All populations possessed similar size-structure and flowering patterns. There was a net dominance of individuals of reproductive stages over non-reproductive individuals and annual mortality was higher for the smaller plants (69-92%) than for the larger ones . Differences were detected between the four populations with respect to reproductive and mortality rates. (4) Values of λ (population growth rate) varied from 0.87 to 1.19. Elasticity analyses revealed that changes affecting size classes 3 and 4 (largest plants) had the highest impact on the populations. (5) Simulations of the effects of harvesting plants showed that 0% (poor growing season) to 16% (good growing season) of the individuals of each size class of a population can be harvested without threatening population survival.