This study evaluated the efficacy of a 14-week aquatic program on physical fitness and aquatic skills for children with autism spectrum disorders (ASD) and their siblings without a disability. Children with ASD (n = 15) and their siblings (n = 15), between 7 and 12 years (8.55 [plus or minus] 2.19 years) participated. In the first 14-week phase, 14 children (group A: ASD, n = 7; siblings, n = 7) received the aquatic program while 16 children (group B: ASD, n = 8; siblings, n = 8) did not. The arrangement was reversed in the second phase of another 14 weeks. Both groups continued their regular treatments/activities throughout the study. Improvements were seen in aquatic skills and physical fitness components except subtest body composition for group A and group B subsequent to aquatic program. The results provide evidence that intervention can be developed to promote motor skills and physical fitness components for children with ASD and their siblings. (Contains 6 tables.)