Alfalfa cultivars with resistance to potato leafhopper (Empoasca fabae, Harris) (PLH) may suffer some yield loss when PLH populations are high. Previous results from studies conducted in cages indicate that the economic action threshold (ET) for PLH was greater for PLH-resistant than for susceptible cultivars, but it is unclear if this holds true under natural field conditions. We established field studies in western Ohio to compare yield of a cultivar with high PLH-resistance (54H91) and a susceptible (54V54) cultivar and to document yield loss in each cultivar relative to the number of naturally occurring PLH present. Yield loss from PLH was calculated as the difference between an early insecticide treatment (12-d regrowth after harvest) and no insecticide treatment. Economic yield loss threshold was considered as 224 kg/ha based on alfalfa value of $US 0.11/kg and insecticide treatment cost of $US 25/ha. The potential for economic loss from PLH is often assessed as the number of PLH in 10 sweeps per 2.54 cm of alfalfa canopy height (PLH index). The recommended ET for PLH in alfalfa corresponds closely to a PLH index of 1. In the susceptible cultivar, the PLH index exceeded this value at the time of early insecticide application in all summer growth cycles during the 3-yr study, reaching values as high as 16. Economic yield loss was observed for the susceptible cultivar in 11 of the 14 summer harvests. In contrast, the PLH index in the resistant cultivar at time of early insecticide treatment never exceeded 5 and in only 4 of 14 summer harvests was economic yield loss measurably greater than the cost of insecticide treatment. We conclude that the ET for alfalfa cultivars with high levels of PLH resistance is a PLH index near 3, or three times the normal ET for susceptible alfalfa.