There is good evidence that natural selection drives the evolution of locomotor performance, but the processes that generate the among-individual variation for selection to act on are relatively poorly understood. We measured prolonged swimming performance, U crit , and morphology in a large cohort (n=461) of wild-type zebrafish (Danio rerio) at ∼6 months and again at ∼9 months. Using mixedmodel analyses to estimate repeatability as the intraclass correlation coefficient, we determined that U crit was significantly repeatable (r=0.55; 95% CI: 0.45-0.64). Performance differences between the sexes (males 12% faster than females) and changes with age (decreasing 0.07% per day) both contributed to variation in U crit and, therefore, the repeatability estimate. Accounting for mean differences between sexes within the model decreased the estimate of U crit repeatability to 21% below the naïve estimate, while fitting age in the models increased the estimate to 14% above the naïve estimate. Greater consideration of factors such as age and sex is therefore necessary for the interpretation of performance repeatability in wild populations. Body shape significantly predicted U crit in both sexes in both assays, with the morphology-performance relationship significantly repeatable at the population level. However, morphology was more strongly predicative of performance in older fish, suggesting a change in the contribution of morphology relative to other factors such as physiology and behaviour. The morphologyperformance relationship changed with age to a greater extent in males than females.