Pollination by bees is required for production of seed alfalfa, Medicago sativa L. The alfalfa leafcutting bee, Megachile rotundata (F.) Hymenoptera: Megachilidae, and the honey bee, Apis mellifera L. Hymenoptera: Apidae, are the most commonly used pollinators. The fungus Beauveria bassiana (Balsamo) Vuillemin has been proposed as a microbial control for Lygus spp. (Hemiptera: Miridae), a common pest of this crop. In laboratory bioassays, we found that adult alfalfa leafcutting and honey bees were susceptible to three strains of B. bassiana. A commercial strain, GHA, was the most pathogenic and virulent, causing approximately 45–80% mortality at a dose of 1 × 105 conidia per bee, and >80% mortality at 1 × 10⁶ conidia per bee. A new strain that is very pathogenic to lygus, 17–41, caused less than 20% mortality in honey bees at 1 × 10⁵ conidia per bee. The alfalfa leafcutting bee was more susceptible than honey bees to all three strains of the fungus. B. bassiana killed almost twice as many alfalfa leafcutting bees, and mortality occurred 1–3 days earlier. Field experiments are needed to further determine the risks this microbial control agent poses to pollinators.