Territorial behaviour is often considered the main mechanism regulating salmonid populations and territory size regarded as the proximate factor that limits abundance, being mainly determined by body size. Despite the spatial requirements of young-of-the-year (YOY) brown trout, Salmo trutta, have been previously established, there is still a gap in the knowledge about the spatial needs of older individuals. Therefore, we aimed to develop an allometric territory size relationship for YOY, juvenile and adult trout. Territory size was inferred from 12-year demographic data and physical habitat simulations performed at 10 pristine sites where populations showed high and stable densities. When compared to previous models in salmonids, results revealed not only interspecific but also intraspecific significant differences in the size of territories used by individuals of comparable size. Interestingly, the explanatory power of the derived allometric territory size relationships was similar to previous models based on individual observational data. It suggested that despite the fact that results obtained from population data cannot explicitly describe territorial behaviour of individuals, they may represent accurately their spatial requirements. Finally, results suggested that territory volume, and not territory area, scaled with body size at a rate consistent with brown trout metabolic rate, the scaling rate being highly constant at any level of simulated proportion of nonterritorial fish in the population. Consequently, if individuals must increase territories to fulfil energetic demands, the allometry of territory volume seems a better predictor of spatial requirements than territory area. Copyright © 2009 John Wiley & Sons, Ltd.