The range of flower-visiting bees of ^*Notocactus polyacanthus, N. sellowii, N. sucineus, and Gymnocalycium denudatum was recorded in the Serra do Sudeste, Rino Grande do Sul, Brazil. 6 out of 22 bee species were noted as frequent visitors to the cactus flowers. For each cactus species we determined the relative frequency of the bees in the flowers, and of stigma contacts during the flower visits. Flower fidelity was determined by analyses of the pollen loads of the females. The effectiveness of the bees as pollinators was recorded for each cactus species. Arhysosage cactorum (Andrenidae), Sarocolletes rugata (Colletidae), and Ancyloscelis fiebrigi (Anthophoridae), specialized solitary bees, are the most effective pollinators of the studied species of Notocactus and Gymnocalycium. Various species of Notocacteae share the same main pollinators. Polylectic and social bees contribute little to the pollination of the cactus species examined in the studied region.