Diversity of passerine birds and mammals was estimated in well-drained areas located at proximity of the hydroelectric reservoir La Grande-3, where natural fire regime still prevails in the absence of forest exploitation. Forest stands were divided up into four post-fire stages: (i) recent burns (4 years old), (ii) shrubs (25 years old), (iii) young forests (50 years old), and (iv) mature forests (greater than or equal to 71 years old). Richness and species diversity were highest in middle stages, in shrubs and young forests. The degree of opening seems to have affected more the composition of bird communities than stand age. Some bird species, typical of shrub stands, in particular white-crowned sparrow (Zonotrichia leucophrys Forster), Lincoln's sparrow (Melospiza lincolnii Audubon), and alder flycatcher (Empidonax alnorum Brewster), appeared after the falling of dead trees, approximately 15 years after fire, and disappeared progressively as forests matured. Deer mice (Peromyscus maniculatus Wagner), moose (Alces alces L.), and black bears (Ursus americanus Pallas) were more common at the beginning of succession, whereas northern red-backed voles (Clethrionomys gapperi Vigors) and caribou (Rangifer tarandus L.) were typical of late stages. Mammal presence was mostly associated to their feeding requirements. Fire creates a mosaic of forest stands through periodic killing of trees in the north of the boreal forest, which contributes to maintain regional wildlife diversity; its suppression would reduce biodiversity.