Specific growth rates of individually reared juvenile three-spined sticklebacks Gasterosteus aculeatus were investigated under laboratory conditions to parameterize a complete temperature-dependent growth model for this species. To test the applicability of experimentally derived optima in growth response rates to natural conditions, the effects of commercial pellets and natural prey on growth rates were investigated. In addition, to test for seasonal effects on growth, laboratory trials were performed in both spring and winter. Growth took place from 5 to 29° C with a temperature for optimum growth reaching a sharp peak at 21° C. Modelled optimal temperature for maximum growth was estimated to be 21·7° C and lower and upper temperatures for growth were estimated to be 3·6 and 30·7° C, respectively. There were no significant differences in growth rates between fish reared on invertebrates or commercial pellets. Seasonal effects on growth were pronounced, with reduced growth rates in the winter despite similar laboratory conditions. On average, 60% higher growth rates were achieved at the optimum temperature in summer compared to the winter. The strong seasonality in the growth patterns of G. aculeatus indicated here reduces the applicability of the model derived in this study to spring and summer conditions.