Extra-curricular physical activity (ECPA) has an important role to play in post-primary schools. The main aim of this study was to examine participation levels of adolescent males (aged 12-15 years) from disadvantaged communities in ECPA and to investigate the impact on the psychological well-being of those who regularly participated. The main psychological areas to be investigated were physical self-worth, perceived physical competence and enjoyment of physical activity (PA). The study also looked to identify barriers to participation in ECPA. Data were generated using self-report questionnaires, activity monitoring devices (pedometers) and focus group discussions. A longitudinal approach was adopted for this study with data being collected at Autumn, Winter and Summer during the academic year 2008/2009. Junior cycle students (N=174, male, 12-15 years of age, mean age 13.59 + .91) from four City of Dublin Vocational Education Committee (CDVEC) schools participated in the study. Overall, N=174 students completed questionnaires, N=146 students recorded pedometer step counts, and N=43 students were involved in focus group discussions. Only 14% of students met Department of Health and Children (DOHC, 2009) guidelines of > 60 minutes MVPA daily. Over the academic year, the average daily step count was 11,891 (SD = + 3,384) and the average number of days per week that students achieved > 60 minutes MVPA was 4.98 (SD = + 1.21). In terms of ECPA participation, 43% of students reported to ‘regularly’ (2 or more times weekly) participate in ECPA, 41% reported to ‘sometimes’ participate (once a week) and 16% reported ‘never’ participating in ECPA. As the rate of ECPA participation got higher, so did the average daily step count and the number of days > 60 minutes MVPA per week. X The students who reported to either ‘sometimes’ or ‘regularly’ participate in ECPA, scored higher on all psychological well-being sub-scales than the students who reported ‘never’ participating in ECPA. An example of this can be seen under perceived physical competence where regular ECPA participants year-long average score was 23.49 (2.36), compared to students who never participated in ECPA who had a score of 14.88 (3.35), F(2,167)=71.0, p<0.01. The focus groups with students, who regularly participated in ECPA, revealed that their main motivations for participation were enjoyment, being with friends and to improve their self-esteem, fitness and skills. These students also recognised the value of participation in ECPA, commenting on how it helped improve attendance and made school more appealing for them. In comparison, the students who never participated in ECPA suggested a lack of perceived competence and peer rejection were their main reasons for non-engagement. These students however, acknowledged the value of ECPA for its regular participants, namely building their confidence and making friends. Strategies suggested by the non-participants that might encourage participation included more positive support from significant others including peers, parents and teachers. The main recommendation from this study is that ECPA in post-primary schools is a potential untapped resource for increasing minutes of PA in adolescent males. The present study has shown that ECPA participation can also have a positive effect on the psychological well-being of adolescent males, in particular, perceived competence, physical self-worth and enjoyment of PA. Future research should repeat this study with a same age female sample and should examine in greater detail, possibly through direct observation, the role of significant others (parents, peers, PE teachers) in the promotion of ECPA participation for adolescents.