We examined the composition of pollen loads of female alfalfa leafcutting bees (Megachile rotundata) at a location where a variety of flowering plants were available, but where alfalfa (Medicago sativa) provided the vast majority of flowers. All females carried alfalfa pollen. Based on counts of pollen grains, pollen loads were greater than 90% alfalfa on three-quarters of the females collected in July and on nearly two-thirds of those collected in August. A single non-alfalfa pollen made up the majority of the loads on just 6% of females. The most common alternative pollen sources were yellow sweetclover (Melilotus officinalis), and Brassicaceae and Chenopodaceae whose pollens we could not distinguish at the species level. In both July and August, the diversity of pollen types on females was positively correlated with flower diversity within 50 m of shelters. This and other studies indicate that M. rotundata is an opportunist capable of exploiting a variety of plant taxa and flowers of different structure.