The highest unexplained mortality of immatureMegachile rotundata(F.) occurs in alfalfa-seed fields. Water-soluble saponins from alfalfa leaves, used to line bee brood cells, seemed capable of causing such mortality. Food masses of bee nests from non-alfalfa areas were injected with extracts of leaves from alfalfa plants selected for high or low saponin content. Mortality after injection with these extracts averaged 56.2 and 37.8%, respectively, and that for controls and uninjected cells averaged 29.0 and 5.9%, respectively. This relationship was consistent for 6 different extracts. Purified alfalfa saponins produced bee mortality directly related to saponin concentration. Suggestions to improve bee survival for alfalfa seed production include: selecting low saponin alfalfa lines, increasing bee populations on clovers, and providing alternative leaf forage near alfalfa seed fields.