A population of Puccinia coronata derived from a bulk collection of urediniospores from oat growing near a hedge of the aecial host, Rhamnus cathartica, was grown for 10 uredinial generations on seedlings of a susceptible oat cultivar in the greenhouse. Frequency of virulence to 14 of 20 single-gene differential oat lines decreased significantly from generation 1 to generation 10. The frequency of virulence to five of the lines did not change, and frequency of virulence to one line increased significantly. The mean number of virulences per single-pustule isolate decreased from 12.5 in generation 1 to 9.1 in generation 10. The apparent conflict between this evidence supporting stabilizing selection against unnecessary virulence and the prevalence of unnecessary virulence revealed in field surveys of oat crown rust is discussed.