Technical Abstract: Five U.S. managed and candidate pollinators for seed alfalfa were compared for the frequencies with which the trip virgin alfalfa flowers, as well as the proportions of these tripped flowers that later set pods and the average count of seeds produced per pod. The latter two measures have never been used to compare alfalfa pollinators, although they should differ rwith the quantity and quality of pollen deposited during tripping. Female alkali bees and alfalfa leaf-cutting bees were the best alfalfa pollinators; honey bees and candidate Osmia aglaea were the least effective, but the differences are entirely attributable to tripping frequencies. Males of the first two species tripped more than half of the flowers that they visited, and should therefore be considered when assessing each species' pollination value for alfalfa.