Current research suggests that a decline in physical activity occurs some time during the adolescent years, but at what specific age is unknown. Determination of the age at which physical activity levels decline and possible contributors to this phenomenon, are needed to increase Australian healthcare knowledge. The participants involved in the study were female adolescents (n = 297) aged 13-15 years from a high school in Southeast Queensland, Australia. The purpose of this study was to: (a) determine mean steps per day, using pedometers, for grade 8 through 10 girls, (b) determine body mass index (BMI) by measuring height (cm) and weight (kg), and (c), using the Children and Youth Physical Self-Perception Profile (CY-PSPP) questionnaire, identify determinants of physical self-worth in Australian female adolescents and their relationship to activity level and BMI. The study revealed that a significant drop-off in pedometer-determined mean daily steps occurred at grade ten, or approximately 15 years of age. A significant inverse relationship between mean daily steps and BMI was found (r = ¯.251, p<.0001). The study also revealed negative correlations between BMI and all self-perceptions (p<.05), except strength, which showed a positive correlation (p<0.05). A positive correlation was found between level of physical activity and all self-perceptions, except Global Self-Worth, which showed no significant difference