The archicture and growth dynamics of Araucaria araucana (Mol.) K. Koch. were studied in its natural habitat (western part of Neuquen Province, Argentina). Individuals growing under various environmental conditions such as in the understorey of forest stands or in open fields, good or poor soils and different rainfall regimes, were observed. In each environment numerous individuals were studied at several stages of development in order to identify the morphological and architectural features of this species. In all conditions, the architecture of both male (producing pollen cones) or female (producing ovulate cones) individuals of this dioecious species is characterised by a strong hierarchy between branching orders. The elementary architecture (i.e. architectural unit) of Araucaria araucana is always composed of three morphologically discernible categories of axes, the nature and relative position of which are not modified qualitatively by environmental factors. Despite a very hierarchical and simple architecture, the individuals of Araucaria araucana display wide range of phenotypic variation due to quantitative differences in the expression of their architectural unit and to a rather opportunistic occurence of the reiteration process. The morphological and architectural bases of these variations are discussed and their adaptive significance and possible role in the survival of this species over time is suggested. Finally, the architectural features of Araucaria araucana are compared with those of other species of the same genus.