Abstract – To investigate differences in behaviour and growth between juvenile brown trout from spatially separated reaches, fish were collected from above and below a weir in Silverstream, New Zealand. Population level differences in mass-specific growth rates and aggression level were examined in tank experiments. Monitoring of mass-specific growth rates found that relative growth rates between fish of different sizes varied between fish from the upstream reach but not between fish from the downstream reach. Large individuals grew faster than smaller individuals from the upstream reach but no effect of relative fish size on growth was found among fish from the downstream population, indicating that social organisation in the two populations differed. When tested in pairs, juvenile brown trout from the downstream population were found to be more aggressive than those from the upstream population. Differences in the allometric relationship between growth rate and fish size and in the levels of aggressiveness appear to be related to the failure to form dominance hierarchies among fish from the downstream reach, a fixed behavioural trait most likely related to prior experience of crowding or different genetic backgrounds.