Powdery mildew of sugar beet (Beta vulgaris), caused by Erysiphe polygoni, was introduced into North American in 1974. Since then, chemical control has been needed. Moderate resistance of a slow-mildewing type is known and has been used commercially. High resistance was identified recently in B. vulgaris subsp. maritima accessions WB97 and WB242 and has been backcrossed into sugar beet breeding lines. These enhanced lines were used as sources of powdery mildew resistance to determine the inheritance of resistance. Analyses of segregating testcross families showed that resistance from both sources is inherited as a single, dominant, major gene. The gene symbol Pm is proposed for the resistant allele. The allelism of the resistance from the two wild beet sources was not determined. Pm conditions a high level of resistance, but disease developed on matured leaves late in the season. This late development of mildew on lines and the slow-mildewing trait in susceptible, recurrent lines tended to obfuscate discrete disease ratings.