Fifty-three collegiate athletes (18 female and 35 male) were recruited from university varsity basketball, football, swim, and wrestling team rosters. Body composition was estimated before the beginning and before the end of sport seasons by using hydrodensitometry to determine the effect of participation in competitive sport seasons on the body composition of collegiate athletes. ANOVAs applied to the estimates of body composition indicated that the interaction of sport team by season was significant for body weight, F (5,47) = 3.2, p less than 0.05; fat weight, F (5,47) = 5.1, p less than 0.01; and fat-free weight, F (5,47) = 2.4, p less than 0.05. Only football players did not change in body composition. Wrestlers lost the greatest amount of body weight (3.4 kg). Female and male basketball players, female and male swimmers, and wrestlers lost significant amounts of body fat (2.6, 2.3, 1.8, 2.7, and 3.6 kg, respectively). Female and male basketball players and female and male swimmers gained significant amounts of fat-free weight (2.0, 1.6, 2.4, and 1.8 kg, respectively). It was concluded that participation in a collegiate sport season could affect changes in fat weight and fat-free weight components of body composition but that the nature of the changes are a function of the demands of the sport of participation.