The purpose of this paper is definitely to record in entomological literature some of the observations and experimental data, which proved beyond a doubt that the common potato leafhopper, Empoasca fabae (Harris), is responsible for the symptoms known as alfalfa “yellows” or “yellow top.” Healthy alfalfa plants in the field and in the greenhouse were caged and infested with leafhoppers collected from the affected alfalfa plants in the field. The initial symptoms on older plants were produced in five days of leafhopper feeding. The young seedlings showed wilting from 24 to 36 hours after the confinement of a single nymph on a young succulent shoot of alfalfa seedling. Prolonged feeding resulted in badly discolored, stunted plants, while the young seedlings were often killed outright. The root system of the infested plants was found to be weakened, which is considered an important contributing factor in the winter killing of alfalfa seedings.