Grayling, Thymallus thymallus and brown trout, Salmo trutta fario populations are substantially limited by availability of suitable habitats. Effect of population density on depth and velocity preference and substrate type on aggressive interactions of wild (age 0+) grayling and brown trout were investigated. The investigations were conducted in allopatric and sympatric situations in an experimental stream that had two substrate environment types: uniform and complex. Allopatric grayling and trout in low population density had strong overlap in depth and velocity preference. High density allopatric grayling indicated a preference for deeper areas. In sympatry and at low population density, the two species had similar depth and velocity preference. However, at higher density grayling selected deeper areas and lower velocities. Both species indicated a positive relationship between size and aggression and were more aggressive at higher density and in uniform substrate environment. In general grayling indicated a higher shift in habitat preference and seemed more sensitive to a change in population density. Therefore grayling is possibly less competitive than brown trout.