Solitary plants of the putative intergeneric cross Oreocereus celsianus × Echinopsis tarijensis were found on two independent occasions in the same general region of S Bolivia (Dept. Potosí). The plants are described and illustrated. They are intermediate in general appearance and flower characters between the supposed parents, and appear to be sterile. The importance of intergeneric and interspecific crosses in the evolution of Cactaceae is briefly discussed, and the relative ease with which intergeneric hybrids can be artificially obtained in cultivation is contrasted with the apparent rarity of naturally occurring intergeneric hybrids. Based on available evidence, the impact of natural intergeneric hybridization on cactus evolution appears to be limited.