Abstract.— A 12-wk experiment was conducted to test the effects of 50, 100, 200, and 300% stocking densities (ventral fish area to bottom tank area ratio) on growth, survival, and aggressive behavior in 2-moold juvenile winter flounder Pseudopleuronectes americanus. Over the course of the experiment, there were no differences (P > 0.05) in growth between treatments at any given week. Survival decreased in all treatments over time, however, proportionately more fish died in the high (200 and 300%) density treatments. At the end of 12 wk, the 300% treatment had significantly lower survival (P < 0.05). There were no differences in the degree of caudal fin damage between any of the treatments (P > 0.05) suggesting that aggressive behavior. such as fin nipping, was not affected by stocking density. However, fish size was inversely related to caudal fin damage. When fish were sorted by length, the two smallest size categories (<2 cm and 2-3 cm) suffered the greatest fin nipping compared to each other (P < 0.001) and all larger size categories (P < 0.001). These results indicate that 50 to 300% stocking densities do not affect growth or aggressive behavior in juvenile winter flounder. Although survival was affected by density, the final yield (weight per treatment) at higher densities was still greater than at lower densities. Our hatchery recommendations are to stock at ≥200% but to monitor fish health carefully. Size grading should be employed to minimize aggression between fish size classes.