Physical activity (PA) behavior correlates with fatness in several distinct populations. Purpose: To determine the relationship between sedentary behaviour, PA and indices of body fatness in adolescents. Methods: Sedentary behaviour and habitual PA in 200 adolescents (102 girls and 98 boys) aged 13–16 years (13 ± 1 years; mean ± SD) were assessed by accelerometry for 5 consecutive days. Time in sedentary activities (<800 counts per minute (CPM) and light >800<3200 CPM), moderate >3200<8200 CPM) and vigorous >8200 CPM) PA were calculated using Puyau cut-points. Indicators of body fatness were assessed using Óskin-folds (ÓSF) (triceps, biceps, sub-scapular and supra-iliac), Body Mass Index (BMI) and waist hip ratio (WHR) and correlated with sedentary and PA of different intensities. Results: Time in sedentary activity was positively correlated with MBI (r=0.44, P<0.001) and ÓSF(r=0.21,P=0.003). CPM and time spent engaged in light, moderate and vigorous intensity activities were negatively correlated with BMI and ÓSF. 16.1% of variation in ÓSF could be explained by gender; R2 = 11.4% and time spent engaged in sedentary activities, R2 = 4.7% ; P<0.05. Conclusion: Time engaged in sedentary activity and PA was a significant predictor of indices of adiposity in adolescents.