Minimal research has examined whether the decline in physical activity (PA) among adolescent girls is associated with chronological age (CA) or biological age (BA). Purpose: To describe the PA levels and perceived barriers to PA of adolescent girls grouped by school grade and maturity status (i.e., early or late maturing) within grades. Methods: Two hundred and twenty-one girls (aged 8-16 yr, grades 4-10) wore an Actical accelerometer for 7 d and then completed a semistructured, open-ended questionnaire on perceived barriers to PA over the 7-d period. Predicted age at peak height velocity and recalled age at menarche were used to assess maturity among the elementary and high school girls, respectively. Maturity and grade group differences in PA were assessed using a MANCOVA and independent sample t-test and barriers to PA using chi-squared statistics. Results: Daily minutes spent in moderate to vigorous PA decreased by 40% between grades 4 and 10. Within grade groupings, no differences in PA were found between early and late maturing girls (P > 0.05). Grades 4 to 6 participants cited more interpersonal (i.e., social) barriers. Grades 9 to 10 participants cited more institutional barriers to PA, primarily revolving around the institution of school. No differences were found in types of barriers reported between early and late maturing girls. Conclusion: Because PA and types of perceived barriers to PA were dependent on grade, future research should work to identify the most salient (i.e., frequent and limiting) barriers to PA by CA in youth. Once reliably was identified, multipronged intervention strategies must be tested for effectiveness 10 help Youth cope with their salient barriers.