Growth hormone (GH) gene transgenesis has allowed the production of salmon with an inherently increased growth potential, on average two to threefold higher compared with daily specific growth rates observed in normal, non-transgenic fish. This difference quickly results in animals of very different sizes at age, and is associated with specific morphological effects and enhanced appetites in transgenic animals. However, less is known of the feeding and antipredator behaviour of GH-transgenic fish, information that can help with predictions of potential ecological consequences of release or escape of transgenic fish into the wild. In a series of experiments, transgenic (T) and normal (N) coho salmon of varying age and size (from 0.5 to 40g, 3.5?21mo) were studied singly, in pairs, and in groups during feeding and simulated predation threat. Vertical position generally did not differ between T and N fry, but at larger size (>4g) T fish remained closer to the surface than N fish both during feeding and predatory attacks, probably as a consequence of inherent differences in feeding motivation and later reinforcement by associative learning. This difference in vertical position was not the result of competition as it remained even after either fish in the pair had been removed. In nature, where predators may attack from above (birds) or below (fish), this kind of behaviour may translate into higher risk of predation, which could increase mortality and lower the fitness of transgenic fish, unless their increased growth rate can compensate for the increased risk-taking.