Blackleg, caused by Leptosphaeria maculans, was first reported on canola (Brassica napus) in North Dakota in 1991. In 2003, L. maculans strains of previously unreported pathogenicity groups (PG) were discovered in the region. Since then, however, little has been known about the prevalence of L. maculans in the state. The objectives of this study, therefore, were to characterize the prevalence of blackleg and of L. maculans PGs in North Dakota. Prevalence was assessed in 2004, 2007, and 2009 in 572 fields. PG determination for 216 L. maculans isolates retrieved from blackleg symptomatic stems during that period was achieved on a set of B. napus differential cultivars. Blackleg prevalence increased from 28% in 2004 to 63 and 74% in 2007 and 2009, respectively. Similarly, the number of fields with blackleg incidences >30% increased from 4% in 2004 to 12 and 23% in 2007 and 2009, respectively. In all years, PG-4 was the predominant group, while PG-2, once predominant, accounted for <2% of isolates. Increase in the prevalence and incidence of blackleg as well as the frequency of virulent PGs over the last 10 years is a serious threat to the canola industry of the region.