Approximalely 15 000 alfalfa leafcutting bees, Megachile rotundata, were released in a O·4-ha planting of hybrid carrots to assess their potential as pollinators. Bees began nesting soon after release and constructed more than 5000 nests in a 7-week period. However, they visited the surrounding carrot flowers only rarely, and used mostiy alfalfa pollen to provision their cells. The predominant insects recorded on carrot flowers were flies, especially syrphids, which were abulldant on both male-fertile and sterile varieties. Flies, rather than M. rotundata, may be a useful alternative to the honeybee for carrot pollination.