Species that occupy the same area and use the same resources must either compete with each other or find ways to minimize competition. For rodents, 1 important resource is nesting sites. In this study I present data from direct behavioral observations in the succulent karoo of South Africa that show aggressive interactions between bush karoo rats (Otomys unisulcatus) and striped mice (Rhabdomys pumilio). Because both species nest in shrubs, the potential exists for interspecific competition for nesting sites. Because of a severe drought in 2003, the bush karoo rat became locally extirpated. As a result, striped mice nested significantly more often in shrubs that contained bush karoo rat nests than in 2001 and 2002, when the population density of bush karoo rats was high. Furthermore, I observed that striped mice never nested in the shrub Lycium cinerum, the favorite nesting site of bush karoo rats, when bush karoo rats were present, but regularly used these nesting sites after bush karoo rats became extirpated. I conclude that striped mice and bush karoo rats compete actively for access to preferred nesting sites in the succulent karoo.