We report a new obligate pollination mu- tualism involving the senita cactus, Lophocereus schottii (Cactaceae, Pachyceereae), and the senita moth, Upiga virescens (Pyralidae, Glaphyriinae) in the Sonoran De- sert and discuss the evolution of specialized pollination mutualisms. L. schottii is a night-blooming, self-incom- patible columnar cactus. Beginning at sunset, its ¯owers are visited by U. virescens females, which collect pollen on specialized abdominal scales, actively deposit pollen on ¯ower stigmas, and oviposit a single egg on a ¯ower petal. Larvae spend 6 days eating ovules before exiting the fruit and pupating in a cactus branch. Hand-polli- nation and pollinator exclusion experiments at our study site near Bahia Kino, Sonora, Mexico, revealed that fruit set in L. schottii is likely to be resource limited. About 50% of hand-outcrossed and open-pollinated senita ¯owers abort by day 6 after ¯ower opening. Results of exclusion experiments indicated that senita moths accounted for 75% of open-pollinated fruit set in 1995 with two species of halictid bees accounting for the remaining fruit set. In 1996, ¯owers usually closed be- fore sunrise, and senita moths accounted for at least 90% of open-pollinated fruit set. The net outcome of the senita/senita moth interaction is mutualistic, with senita larvae destroying about 30% of the seeds resulting from pollination by senita moths. Comparison of the senita system with the yucca/yucca moth mutualism reveals many similarities, including reduced nectar production, active pollination, and limited seed destruction. The in- dependent evolution of many of the same features in the two systems suggests that a common pathway exists for the evolution of these highly specialized pollination mutualisms. Nocturnal ¯ower opening, self-incompati- ble breeding systems, and resource-limited fruit pro- duction appear to be important during this evolution