Growth rates of age-0 brown troutSalmo truttaduring the growing period (May–September) were estimated in nine consecutive year-classes in three sites of Arroyo Chabatchos (Esva River basin, northern Spain). Observed growth rates were highly correlated with average water temperature, which explained 48, 68 (90% excluding the severe drought of 1989), and 89% of the interannual variation in respective growth rates at the three sites studied. Observed growth rates varied from those predicted by a model by Elliott and others, which predicted higher-than-observed growth rates in winter and negligible growth in summer. Observed growth rates were consistently higher than those predicted in two sites, and the only four instances (out of 27 possible) of lower-than-predicted growth rates occurred in fish at the site at which observed growth was slowest. As expected from the model, the predicted growth rates were also correlated with water temperature, but the slopes of the observed and predicted regressions differed significantly. The faster growth at higher temperatures exhibited by Chabatchos brown trout may be a local adaptation of the thermal regulation of growth. This adaptation does not seem to have induced a shift to a higher optimal growth temperature but only a slower reduction of growth rates with increasing water temperature. This allows Chabatchos brown trout to maintain growth that is greater than other brown trout at comparable temperatures, enabling these fish to achieve larger sizes in most growing seasons.