In the Sonoran Desert the giant columnar cactus, Carnegiea gigantea, often shows a highly clumped distribution because of its frequent establishment beneath tree canopies. An apparent consequence of this distribution pattern, where several Carnegiea are associated with a single tree, is intraspecific competition for limited water. The relative amount of stored water, rate of water uptake, apical growth, and reproductive potential of individual Carnegiea are all lowered as a function of increased levels of intraspecific crowding. These ecological interactions may ultimately affect survivorship and influence patterns of distribution and abundance of this cactus species.