Variations of density and of population structure ofOctodon degusin the area of National Park “La Campana”, two habitats with different grade of intervention during the time of breeding in two consecutive years, are shown. The density of this rodent is in inverse relation to the degree of alteration of the habitat, showing its sensibility to the changes of the habitat. At the end of winter (August/September) the population would have its lowest density. Its maximun expression is obtained at the beginning of summer (December) coinciding with the breeding period. The relationship between immature adult from one year to the other shows a close relation. However, the variations observed during the same year are due to the recruit of the immature of the two offsprings that the rodent produces. When demand is greatly reduced from February to May, the palm fruit is an important nutricional help. But it has not been well defined quantitatively. The other members of the community of micro‐mammals show the same tendency ofOctodon degus:a low density at the end of winter that increases as spring advances.