The influence of the peer group on young people's achievement motivation has been highlighted in the literature as an area that needs examination (e.g., Harwood & Swain, 2001). To this effect, a new measure of youngsters' perceptions of the peer motivational climate (Peer Motivational Climate in Youth Sport Questionnaire; PeerMCYSQ) was developed and tested across three studies. In Study 1, exploratory and confirmatory factor analyses (CFA) with 431 athletes between the ages of 11 to 16 years suggested that the PeerMCYSQ had 6 factors that could also be subsumed into 2 higher order factors (TaskInvolving climate: improvement, relatedness support, effort; Ego-Involving climate: intra-team competition, normative ability, intra-team conflict). In Studies 2 and 3 the 6-factor solution and the corresponding hierarchical one were tested using CFA with two independent samples (N = 606 and 495, respectively) of similar age. The results showed that the 6-factor model was problematic and that a 5-factor solution should be preferred instead. Further support to the 5-factor model was provided with hierarchical and multilevel CFAs. Suggestions for further research on peer motivational climate are discussed.Key Words: peer influence, scale development, confirmatory factor analysis, achievement goal theory Youth sport involves the participation of young people in sport activities organized and/or supervised by adults. Such activities are considered as some of the most pervasive and popular pursuits for boys and girls in many countries around the world. Peer interactions and relationships are particularly important in youth sport and can contribute to the quality of youths' overall experiences in this context (Smith, 2003). The literature on peer relationships in youth sport has rapidly increased in size and diversity (for comprehensive overviews of the literature, see Brustad, Babkes, & Smith, 2001;Smith, 2003). Issues such as peer acceptance and its relationship to physical competence, friendship in sport, information sources for competence evaluation, and the links between peer relationships with affect and moral development are some of the topics that have attracted research interest in this area. For example, research has shown that peers become progressively Motivational Climate in Youth Sport / 433 more important as significant others as children grow older. Young children under 10 years of age rely more on adult feedback to judge their competence compared to those in late childhood and early adolescence, whose central source of competence information is peer comparison and feedback (Horn & Weiss, 1991).Research on how peer interactions affect children's motivation in youth sport is limited. Studies have shown that peer acceptance and friendship are related to correlates of motivation, such as high levels of commitment and enjoyment and lower levels of anxiety (e.g., Kunesh, Hasbrook, & Lewthwaite, 1992;Weiss & Smith, 2002). However, only a handful of studies have examined how peer influence transmits and fosters achieve...