Twelve fungal strains belonging to three different subdivisions were assessed for their ability to decolourize mechanical pulp derived from Douglas-fir (Pseudotsuga menziesii). Pulp treated with several fungal strains, including Phanerochaete chrysosporium, showed a slight brightness improvement before and after peroxide bleaching. However, pretreatment by many white-rot and brown-rot species caused a significant decrease in brightness. The strains were also examined for their impact on paper strength and brightness reversion. Treatment by several species resulted in substantial strength losses, while only minor beneficial effects on the prevention of brightness reversion were observed. The previously reported beneficial effects on paper strength properties after biopulping of wood chips were not achieved after a fungal treatment of a mechanical pulp.