Although pollination effectiveness is a central process underlying the evolution of plant and pollinator traits, it is difficult to measure and has rarely been reported for a diverse spectrum of visitors under natural conditions. We measured the effectiveness of all common flower visitors to Asclepias tuberosa (butterfly weed) at a site in southeastern Arizona, in terms of visitation rate, per-visit rate of pollinia removal and insertion, and pollinia load. Bombus and Apis (Hymenoptera) were the most effective pollinators, counter to predictions that A. tuberose is butterfly-pollinated. We also documented large differences between 2 yr in the pollination effectiveness of visitors, primarily due to changes in visitation rate. Bombus were the most frequent and effective pollinators in 1992. In 1993, Apis were equivalent to Bombus. Battus (Lepidoptera) were the second most effective pollinators in 1992, but were scarce in 1993. Thus, conclusions about the identity of effective pollinators based on floral traits, casual observations of visitation, or even precise measurement of effectiveness in a single season are potentially suspect. We compare our results to those of previous studies of Asclepias pollination.