Reproductive aspects of a peripheral population of Galaxias maculatus are described and the relationship with the physiology and handling of energetic reserves under marginal environmental conditions is investigated. The G. maculatus population of Tierra del Fuego has an extended reproductive season, with differences in timing and duration compared to other populations of continental Patagonia, New Zealand and Australia. Elevated gonadosomatic indexes (IG) were observed during this period in both sexes (maximum IG = 33.49% males; 35.94% females). The high abundance of mature males (with high IG values) on the spawning grounds during the reproductive season suggests that they were waiting for the return of the mature females. Larger females reached total maturation at the beginning of the reproductive season, whereas the size of maturing females diminished toward the end (mean TL = 96 mm, October; 70 mm, February). Both sexes showed an extremely high investment in reproduction, reaching a maximum IG of about 35% for both sexes. At the beginning of the reproductive season females reached the maximum median IH (3.37%) and males the minimum (0.96%), suggesting differences in the role of the liver in the management of energetic reserves during sexual maturation. The variation in the fat index (IF) suggests that fat reserves were used to survive winter (maximum median IF > 1%, autumn; minimum about 0.2%, spring).