The structure of different standes in an old growth Araucaria - Lenga forest were analyzed with respect to the distinct development stages. The description of the stands was made mainly on the basis of their horizontal and vertical profiles, diametric frequency distributions, basal area and crown cover. These parameters were individually analyzed, for each generation within the stands. The dynamics of the natural development cycle was measured through the observed change in structure of the samples taken in the stands, which in turn were previously ranked according to the ages of the trees. The stages that conform this cycle, a description of which is in this paper, are as follows: 1.— Break-down phase and regeneration. 2.— Optimal growth phase. 3.— Aging phase.En un bosque virgen de Araucaria - Lenga se analizó la estructura de bosquetes en distintas etapas de desarrollo. La caracterización del bosque se hizo principalmente a base de perfiles horizontales y verticales, distribuciones diamétricas, áreas basales y coberturas, para lo cual se diferenciaron en cada bosquete las generaciones de árboles presentes. Se interpretó la dinámica del ciclo de desarrollo natural a través del cambio de estructura de las parcelas, previamente ordenadas de acuerdo a la edad de los árboles. Se describen las fases que constituyen este ciclo, las cuales son: 1.—Fase de desmoronamiento y regeneración. 2.—Fase de crecimiento óptimo. 3.—Fase de envejecimiento.