The pollination ecology of Opuntia cf. macrocentra (Cactaceae) was studied throughout the Trans-Pecos region of Texas, March-May, 2000 and 2001. Floral phenology was investigated, and insect visitors to flowers were monitored for pollinating behaviors and captured for identification. Pollen morphology was examined for each studied population of O. cf. macrocentra and compared to pollen on bodies of collected insects. Breeding tests of geitonogamy and autogamy demonstrated self-incompatibility. Like other Opuntia species, O. cf. macrocentra flowers are diurnal, usually lasting only one day. Inclement weather may prolong blooming beyond one day. Flowers are protandrous: anthers dehisce in the morning within a few hours of anthesis. The peak of stamen thigmotropic response corresponds with stigma receptivity. Stigmas are receptive in the afternoon; receptivity may be accompanied by a color change. Floral tepals change color as they age. Numerous individuals of Carpophilous spp. beetles were observed consuming floral parts and mating within flowers. No beetle contact with stigmatic surfaces was observed, nor did collected Carpophilous beetles carry significant amounts of pollen on their bodies; therefore, beetles are not significant pollinators of Opuntia cf. macrocentra. Foraging ants gathered pollen in their mandibles and traveled up and down styles, but they did not deliver pollen to the stigmatic surfaces. Bees frequently visited the flowers of Opuntia cf. macrocentra and were consistently observed upon stigmas. Bees also carried copious Opuntia pollen loads on their bodies. The most effective pollinators were Diadasia rinconis (Apidae), Lithurgus littoralis (Megachilidae), Ashmeadiella opuntiae (Megachilidae) and Melissodes tristis (Apidae). Secondary pollinators include Apis mellifera (Apidae), Lasioglossum (Dialictus) pruinosiformis, and Lasioglossum sp. (Halictidae). Of these, Diadasia rinconis, Lithurgis littoralis, and Ashmeadiella opuntiae are oligolectic, only collecting pollen from Cactaceae. Apis mellifera, M. tristis, and Lasioglossum (Dialictus) pruinosiformis are polylectic.

Key words: bee pollination, Cactaceae, Opuntia macrocentra, pollination ecology