Exercise training in fish leads to enhanced growth, mostly in salmonids, but also in gilthead sea bream (Sparus aurata). Most of the published studies have involved juveniles or adult fish, but little is known about the effects of exercise on early stages of fish growth. Our study aimed to examine the effect of sustained swimming on the growth of gilthead sea bream fingerlings, by analysing white muscle cellularity and capillarisation. Two groups of fingerlings were compared: the exercise group (EX), forced to swim at five body lengths·s−1, and the control group (CT), maintained under still water flow and showing only voluntary swimming. After 5 weeks the EX group had a significantly higher body weight (CT: 17.5 ± 0.46; EX: 20.3 ± 0.38 g, p < .001) without significant differences in muscle-somatic index (CT: 34.9%; EX: 37.3%). The white muscle of EX fish showed significant reductions in fibre cross-sectional area (FCSA) and fibre perimeter (FPER), by 21% (p < .05) and 10% (p < .0.5) respectively, with no differences in fibre circularity. The number of small fibres (with FCSA <2000 μm2) was significantly (p < .05) higher in the EX than in the CT group. Therefore, the distribution of FCSA demonstrated an increase in hyperplasic processes in EX fish. Total muscle capillary density (CD) and the capillarisation of individual fibres (expressed as the number of capillaries per unit FCSA) of the EX group also increased significantly (p < .05 and p < .001, respectively). In summary, sustained aerobic exercise in the early stages of life of gilthead sea bream enhances body growth, increasing total muscle mass by hyperplasia and determining a more aerobic muscle phenotype by increasing individual fibre capillarisation. •Gilthead seabream fingerlings enhance growth and total muscle mass under moderate and sustained exercise.•In exercised muscle, a greater number of small fibres base early growth on hyperplasia processes.•Sustained swimming increases white muscle fibre capillarization