Technical Abstract: We compared nectar secretion rates and bee guilds of yellow star-thistle, Centaurea solstitialis on Santa Cruz Island (USA) and the Northeast Aegean Island of Lesvos (Greece). This plant species is non-native and highly invasive in the western USA but native to Eurasia (including Lesvos). "Nectar flow" was assessed by measuring nectar volumes in florets of flower heads covered with mesh bags (preventing visitation by bees); "nectar standing crop" data were taken from open (unbagged) flower heads to which all bees could gain access. We censused bees at C. solstitialis during comparable periods on both islands and determined the bee guild composition of the plant on Lesvos. Significant differences in nectar levels occurred between bagged and unbagged florets at each locale, especially during the period that pollinators were most common. Nectar flow and nectar standing crop volumes were lower on Lesvos than on Santa Cruz Island. The bee guild diversity at Lesvos was higher relative to Santa Cruz Island. Surprisingly, however, honey bees were not recorded during our monitoring periods on Lesvos.