We compared mean body weights, nutrient reserves (fat and protein), and an index of feeding of lesser snow geese arriving at and leaving southern James Bay in spring and fall 1976. Feeding increased in spring and decreased in fall. Body weight and protein of adults increased in spring, and fat reserves were maintained. The protein increase of adult females was equivalent to the protein in one egg. Ovarian follicular development in adult females appeared to proceed as reserves became available. Juvenile males had completed body growth by the end of the fall staging period but juvenile females had not. Fat and protein reserves of adults arriving on James Bay in the fall were significantly larger than those of adults after wing molt on the breeding grounds. Fat reserves of adults and juveniles increased significantly during the fall; body weight of juvenile females increased also. The fat gain greatly increases the theoretical maximum flight range of the geese and we suggest it is an important determinant of normal fall migration.