One of the most destructive pathogens of canola (Brassica napus L.) is Leptosphaeria maculans (Desm.) Ces. & De Not., which causes blackleg disease. This fungus produces strains with different virulence profiles (pathogenicity groups, PG) which are defined using differential cultivars Westar, Quinta and Glacier. Besides this, little is known about other traits that characterize these groups. The objective of this study was to characterize the aggressiveness of L. maculans PG 2, 3, 4, and T. The components of aggressiveness evaluated were disease severity and ability to grow and sporulate in artificial medium. Disease severity was measured at different temperatures on seedlings of cv. Westar inoculated with pycnidiospores of 65 isolates. Highly significant (α=0.05) interactions were detected between colony age and isolates nested within PG’s. Warmer temperatures resulted in increased severity by all PG. These studies allowed the identification of aggressive isolates to be used for screen breeding lines for resistance.