The purpose of this study was to test the hypothesis that infection of jack pine (Pinus banksiana Lamb.) by jack pine dwarf mistletoe (Arceuthobium americanum Nutt. ex Engelm.) increases with increasing tree age and decreases with increasing tissue age. One-, 2-, and 3-year-old tissues of 3-, 5-, and 7- year-old jack pines and 1-, 4- and 8-year-old tissues of 12-, 17-, and 22-year-old jack pines in Belair Provincial Forest were inoculated with seed of A. americanum in September of 1992 (year 1) and 1993 (year 2). Overwinter and postwinter seed removal, fungal and insect damage, and infection success were monitored from the time of inoculation to harvesting of inoculated branches. In years 1 and 2, overwinter seed displacement was 12.2 and 30.6%, while postwinter loss was 28.8 and 22.2%, respectively. Seed germination ranged from 14.3 to 38.1% and from 3.1 to 17.5%, respectively, in years 1 and 2. Infection success varied from 2.0 to 35.0% (year 1) and from 0.0 to 13.0% (year 2). Lower mean daily temperatures in January and February (p < 0.001) were hypothesized to have killed more seeds and thereby reduced infection success in year 2. Infection success did not increase with increasing tree age (year 1: p = 0.188; year 2: p = 0.807) in either year of the study. Infection success increased with increasing tissue age in year 1 (p < 0.001) but not in year 2 (p = 0.358). We rejected the hypotheses that susceptibility to infection by A. americanum increases with increasing tree age or decreases with increasing tissue age of jack pine. Infection success appears to be primarily dependent upon seed displacement caused by wind, snow, or rain.