SummaryApproximately 15 000 alfalfa leafcutting bees,Megachile rotundata, were released in a 0·4-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 carrol flowers only rarely, and used mostly alfalfa pollen to provision their cells. The predominant insects recorded un carrot flowers were flies, especially syrphids, which were abundant on both male-fertile and sterile varieties. Flies, rather than M.rotundata, may be a useful alternative to the honeybee for carrot pollination.