The loculoascomycete Leptosphaeria maculans (anamorph: Phoma lingam) causes blackleg of Brassicas, including Brassica napus (canola or rapeseed). This fungus probably comprises several morphologically similar species; taxonomic relationships between them are being clarified and nomenclature is being revised. The pathotype ("A" group) responsible for major economic losses to canola has been studied in more detail than other members of this species complex and is the focus of this review. L. maculans is haploid, outcrossing, can be transformed, and has a genome size of about 34 Mb. Preliminary genetic and physical maps have been developed and three genes involved in host specificity have been mapped. As yet, few genes have been characterized. Chemical analysis of fungal secondary metabolites has aided understanding of taxonomic relationships and of the host-fungal interaction by the unraveling of pathways for detoxification of antimicrobial phytoalexins. Several phytotoxins (host and nonhost specific) have been identified and a complex pattern of regulation of their synthesis by fungal and host metabolites has been discovered.