The effectiveness of precommercial crop tree thinning was investigated by thinning crop trees of paper birch, yellow birch, and sugar maple to two intensity levels at 8 years of age. Analysis of covariance on post thinning growth measurements taken 7 years later indicated a significant increase in diameter and crown-diameter growth, with the pattern of response varying by species. The study suggests that a uniform crop-tree thinning treatment for a stand composed of species with varying tolerances will rarely produce the best response for all the species.