Honey bees (Apis mellifera L.) begin visiting flowers of the saguaro, or giant cactus (Carnegiea gigantea (Engelm.) Britt. & Rose), at dawn, collecting pollen for a couple of hours and nectar until shortly after noon. A single flower may produce 3 ml. or more of nectar, containing about 25% of sugar, and a dozen or more bee loads of pollen. Honey bees are the predominant visitors although other bees also visit the flowers. The latter visit flowers on different trees freely. Honey bees tend to confine their visits to individual flowers or branches (arms) of one plant when nectar is abundant, but visit more widely scattered flowers when competition is strong. The effectiveness of honey bees in pollination of saguaro appears to depend on their population in proportion to the number of available flowers.