We investigated the effect of swimming training in rats on morphological characteristics, hypertrophy levels and metabolic and contractile adaptations of different fiber types in rat the rectus abdominis muscle. Rats were randoverly assigned to one of three groups (N = 5 each): a) swimming training for 1 h, twice a week (2x/w), b) 1 h five times a week (5x/w) for 9 consecutive weeks, or c) without any swimming training (Control). Body weight increased in 2x/w rats and decreased in the 5x/w ones. Muscle fiber diameters increased in both trained groups, with higher values in the 2x/w group. Aerobic exercise increased slow oxidative (SO) + fast-oxidative-glycolytic (FOG) fibers and decreased fast glycolytic (FG) fibers. In addition, SO fibers were increased and FOG + FG fibers were decreased in trained rats. Small and angulated atrophic fibers were also observed. These results corroborate that swimming exercise increases aerobic metabolism and thus oxidative and low contraction fibers. However, the used protocol induced, to a variable degree, changes in the muscle fiber morphology