Although physiological adaptations to aerobic endurance training differ in many ways in terms of age, gender and genetic factors, aerobic endurance training may result in decreased body fat, increased respiratory capacity, maximal oxygen uptake and working efficiency of the heart (Kraemer, 2000). The purpose of this study was to determine the effects of a ten-week aquatic exercise and swimming program on the body composition and under water working capacity of mentally retarded children. Nine trainable and seven educable children from the Ankara Shelter and Rehabilitation Center participated in this study. The mean age of the children was 12.22±0.49 years in the trainable group (n=9) and 14.71±0.52 years in the educable group (n=7). The training program consisted of a 40-minute swimming and water exercise session, twice weekly for ten weeks. The study included both a pre-test and post-test measuring body composition. In addition, heart rate and running speed in the water were measured to determine the underwater working capacity. Percentage body fat and fat free mass were measured via skinfold measurements (biceps, triceps, subscapula, chest, suprailiac, abdominal thigh, and calf). The Mann Whitney U test was used to compare the two groups and the Wilcoxon Matched Pairs Signed-Rank test was used within each group. A comparison between the results of the pre-test and post-test indicated that there was no significant effect on the body fat and fat free mass of both trainable and educable groups parameters within and between the groups (p>0.05). Although both groups improved significantly within the group in heart rate and running speed in the water (P<0.05), there was no significant effect on between the groups (p>0.05). The results of the study show that these exercises have no significant effect on the subjects’ body composition. Parallel to findings of Ruoti et. al., (1994), this study concludes that a longer training period is needed to have an impact on body composition via water exercises and swimming. However, similar to literature (Baltaci et. al., 1996; Yilmaz et. al., 1996) this study found that water exercise and swimming have viable effects on cardiovascular endurance. This study has several limitations, including the characteristics of the participants, intervention, tests, and the measurements that were applied. Further studies should address the effects of different settings and types of physical activities on the body composition and underwater working capacity of children with mental retardation.