Seasonal growth of Galaxias maculatus in Tierra del Fuego, at the southern limit of its distribution, can be explained by the variation in extrinsic factors, and experiments with fasting in summer conditions would allow researchers to focus on the potential feeding/fasting conditions independent of other seasonal cues, and thus distinguish the effect of fasting from other winter factors influencing fish growth. The study presents information on muscle growth in G. maculatus as well as the results of an experiment evaluating the effect of fasting on the growth rate, total muscle growth and energy density in this species under summer-like conditions. A uniform pattern of muscle fibres suggests the absence of mosaic hyperplastic growth in G. maculatus. Hyperplasic growth was completed before puyen reached 65 mm total length, and hyperplasia cessation was correlated to the transition from the juvenile to the adult stage. In the experimental fish, where hypertrophic growth was the unique process responsible for adult muscle growth, fasting decreased mass, condition and energy density of the individuals, and stopped hypertrophic growth. These results are proof that reduced food consumption in southerly puyen populations during winter might contribute to the seasonality of the growth previously reported for this species. A provisional summer growth rate of 2–3 mm/month is suggested for populations inhabiting the southern limits of this species' distribution. G. maculatus is thus proposed as a useful model species to study hypertrophic muscle growth.