the competitive relationship between jack pine (finns banksiana lamb.) and trembling aspen (populus tremuloides michx.) was studied in an 8-year-old mixed stand in north central ontario, canada. during the summer of 1984, a total of 298 subject trees were selected at random within the stand. half of the study trees were jack pine, the other half aspen. from the trees neighbouring each subject tree, competitor trees were identified by means of a modified horizontal point sample centered on the subject tree. the data include a wide range of conditions with respect to both stand density and composition. subject trees were measured for initial size (stemwood volume, stem radius at stump height and height) and the 1983 annual increment of the same size attributes. live crown ratio at the time of sampling was also measured. the current annual increment data were analysed with respect to initial subject tree size and competitive environment by means of multiple linear regression. the objective was to investigate a hierarchy of mutually exclusive hypotheses regarding the nature of the competitive interaction between the 2 study species. the results indicate that jack pine and aspen responded differently to competition, and that the competitive effect of the jack pine component was different than that of the aspen component when both species occurred in a mixture. specifically, the annual volume increment and annual radial increment of aspen subject trees was affected only by the aspen component of the competing stand. the annual volume increment and annual radial increment of jack pine subject trees, on the other hand, were affected by both the jack pine and aspen components of the competing stand. although the study is of limited scope, it does provide some interesting insights into the possible dynamics of mixed stands of these important, and commonly associated, boreal species. the work also gives some indication that the competitive effect of aspen on jack pine may be sufficiently strong to warrant the consideration of silvicultural control in situations where jack pine is the crop species.