Objective: This study evaluated the ability of delinquent peer affiliation to mediate the effects of multidimensional treatment foster care (MTFC; Chamberlain, 2003) on girls' delinquent behavior. Method: This study used a sample of girls from 2 cohorts (N = 166; M = 15.31 years old at baseline, range 13-17 years; 74% European American, 2% African American, 7% Hispanic, 4% Native American, 1% Asian, and 13% mixed ethnicity) and measures of delinquent behavior, including general delinquency, number of criminal referrals, and number of days in locked settings. As the mediator, we used self-reports of affiliation with delinquent peers. Our analytic plan specified an intent-to-treat analysis within the framework of a randomized controlled trial comparing MTFC with traditional community-based group care. Results: Random assignment to the MTFC program reduced girls' number of criminal referrals and number of days in locked settings at 24 months. The MTFC condition also reduced girls' exposure to delinquent peers at 12 months, which in turn reduced levels of all forms of delinquent behavior at 24 months; indirect effects were statistically significant. Conclusions: Reduction in exposure to delinquent peers mediated MTFC effects on the number of criminal referrals and number of days in locked settings; delinquent peers also served as an intervening variable between MTFC and self-report delinquency, suggesting that, by reducing contact with delinquent peers, MTFC helped to encourage lower levels of self-report delinquency. Existing prevention and intervention programs targeting similar populations may benefit from increased attention to reductions in delinquent peer affiliation in female samples. (Contains 2 figures and 4 tables.)