Adult male Branta canadensis maxima were collected from a nonmigratory feral population during their premolt, molt and postmolt phases. Lean dry weight of the pectoralis muscle decreased significantly (p less than or equal to 0.0001) during molt, as a result of disuse atrophy. Histochemical analysis revealed that the region of the pectoralis muscle sampled consisted of Red (fast-twitch oxidative-glycolytic) and White (fast-twitch glycolytic) muscle fiber types, in an approximate ratio of 9 to 1. There was no significant (p = 0.1238) difference in the relative percentages of the two fiber types during the three periods of study. There was, however, a significant decrease in mean cross-sectional area of both Red (p less than or equal to 0.0194) and White (p less than or equal to 0.0001) fibers during molt. Red and White fiber areas were strongly correlated with each other during molt (r2 = 0.76, p = 0.0010) and postmolt (r2 = 0.70, p = 0.0052), but not during premolt (r2 = 0.02, p = 0.7626). The latter finding may be related to fiber-type specific hypertrophy in premolt breeding males. Analysis of ultrastructure revealed that there was a significant (p = 0.0003) decrease in the mean myofibrillar cross-sectional area, and a significant increase in both the density (p = 0.0227) and total number (p = 0.0058) of myofibrils within the muscle fibers of the molting birds. These results indicate that the myofibrils split longitudinally during molt-associated disuse atrophy.(ABSTRACT TRUNCATED AT 250 WORDS)