Common brushtail possums (Trichosurus vulpecula) are environmental and agricultural pests in New Zealand. This article investigates their denning behavior and the response of such behavior to density reduction. Brushtail possums prefer to den in trees with large diameters. Individuals in the populations recovering from density reduction used more den sites than those in preremoval populations. Simultaneous den sharing was uncommon in both preremoval and recovering populations. Sequential den sharing was common and occurred mostly between females and males. Den-sharing frequencies were not reduced by population reduction. Our results indicate that transmission of diseases between brushtail possums through den sharing is not likely to be greatly affected by density reduction.