This paper is based on the investigation of 173 lochs in Scotland from which a total of over 4700 brown trout were examined. Twenty‐four of the lochs, all in northern Scotland, and their trout populations, were selected for detailed comparison in an attempt to identify environmental factors that might affect the growth rate of trout. The lochs probably represented the full range of main standing water habitats in the region‐ranging from a saline loch in the Outer Hebrides to one in a sub‐arctic environment high on the Cairngorm plateau in the Central Highlands. The study indicates that for practical purposes the growth rate of trout is negatively correlated with population density. Some suggestions for improving brown trout lochs, based on the findings of this paper, are inc