All isolates of Septoria avenae Frank f. sp. avenae studied were pathogenic on selected varieties of oats and there was no evidence of a differential reaction. The virulence of the isolates varied greatly and this variability was inherent in all groups of isolates. The dimensions, septations, and numbers of macrospores produced by the isolates on oat-leaf agar also varied greatly but there was no correlation between these variable characters and the disease rating of the isolates.