Witches'-brooms were occasionally observed on black spruce (Picea mariana (Mill.) B.S.P.) under jack pines (Pinus banksiana Lamb.) heavily infected with Arceuthobium americanum Nutt. ex Engelm. approximately 100 km northeast of The Pas, Manitoba. Brooms rarely exceeded 20 cm in diameter before dying. The endophytic system of A. americanum was present in twigs of the witches'-brooms, but neither aerial shoots nor basal cups were found, suggesting an incompatible host-parasite relationship. Blue spruce (Picea pungens Engelm.) is apparently also incompatible, while Engelmann spruce (P. engelmannii Parry) and white spruce (P. glauca (Moench) Voss) are infected and aerial shoots are produced (F. G. Hawksworth, personal communication). A. americanum should not impact the black spruce resource. Black spruce should be considered a rare host for A. americanum.