In 1997 several seed companies released alfalfa products that were marketed as resistant to the potato leafhopper, (Empoasca fabae Harris), the key pest of this crop in the Midwest and northeastern United States. The objectives of this research were to investigate the mechanism of insect resistance in leafhopper-resistant alfalfa and to determine if potato leafhopper-resistant alfalfa would require updated pest-management guidelines. It was determined that an antixenotic mechanism functioned at a plant or stem level, and without a choice, leafhoppers could feed as much on resistant alfalfa compared with susceptible alfalfa. Antixenosis was largely a function of insect behavior. The antixenotic mechanism detected in lab studies was not detected at the field-plot scale. The densities of potato leafhopper adults and nymphs were similar among plots of one susceptible alfalfa and four leafhopper-resistant alfalfa during three years of sampling.