Incidence of forest areas severely infested with dwarf mistletoe (Arceuthobium americanum) was assessed from the air on 140,000 ha of mature jack pine (Pinus banksiana) in commercially important forests in Manitoba. Approximately 12,000 ha or 9% of the area surveyed was infested. This dwarf mistletoe kills jack pines and reduces crown closure. Crown closure was used as a surrogate for stand density in computing stand timber volume. If crown closure was 51–70% in the absence of dwarf mistletoe, the infested area could have supported as much as 53% more volume than was present. If crown closure exceeded 70% without dwarf mistletoe, timber volume would have been 70% greater than that present. The volume reduction caused by dwarf mistletoe represented 4–8% of the merchantable volume on the surveyed area.