Sport is an important developmental context for children, and experiences involving parents, coaches, and peers affect a variety of important psychosocial outcomes, including motivational processes. Linking concepts from achievement goal theory with motivational constructs in self-determination theory, this study examined relations between the motivational climate created by parents and both the nature and changes in sport-related motivation in young athletes, using Grolnick and Ryan’s autonomous regulation index, which summarizes the relative strength of intrinsic and extrinsic forms of motivation. We followed a sample (N=308) of 9-14 year old swim club athletes during a 32-week season, measuring their reports of the parent-initiated motivational climate as well as autonomous regulation at the beginning of the season, at midseason, and at the end of the season. Cross sectional analyses at each point revealed that children whose parents created a mastery climate, which defines success in terms of enjoyment of the activity, self-improvement, and effort, reported higher levels of autonomous regulation (intrinsic motivation) than did those whose parent created an ego climate that emphasized winning, avoidance of mistakes, and ability comparison with others. In contrast, ego climate scores were positively related to extrinsic motivation scores. Girls exhibited higher autonomous regulation than did boys. An extreme-group longitudinal analysis showed that children exposed to a strong mastery environment exhibited higher autonomous regulation and increased in autonomous regulation from mid-season to late-season, whereas an ego-climate group decreased in internal regulation during this interval.