Background: To promote regular physical activity (PA) among children and adolescents, authors recommend that physical education (PE) teachers offer their students programmes that would allow them to be physically active outside PE classes. However, such programmes are rarely rigorously assessed and it is recommended that further studies be undertaken to better understand characteristics of interventions that render children and adolescents more physically active. Aims: The purposes of this study were (a) to examine the effectiveness of a PA intervention, the team pentathlon, and (b) to examine whether the impact of team pentathlon was different for boys and girls. Method: Four primary school PE teachers and eight 5th grade elementary classes (two for each PE teacher: one experimental and one control) took part in the study. Overall, 86 students (44 girls; 42 boys) experienced team pentathlon in addition to their regular PE classes whereas 82 students (41 girls; 41 boys) simply attended their regular PE classes. The research design included: (a) a 3-week baseline measure of the physical activity volume (PAV) for both the experimental group (EG) and the control group (CG); (b) the implementation of a 5-week intervention programme and weekly PAV measures for the EG (weeks 4 to 8) and periodic PAV measures for the CG (weeks 4, 6 and 8); (c) a 2-week recess for both groups (weeks 9 and 10); (d) PAV measures for both groups during weeks 11 and 12. PAV was computed in terms of pentathlon hours (PH), meaning the actual number of activity hours reported by students, corrected according to the intensity (energy cost) of the practiced physical activities. A repeated measures ANOVA was performed to see if there was a difference between the experimental and control groups, between boys and girls and between the four repeated measures. Non-parametric analyses were performed to see if there was any significant difference between reported volumes of physical activity for each PE teacher's classes. Results: Overall the results showed that control students' level of activity remained stable while experimental students were significantly more physically active at the time of the pentathlon implementation as well as three weeks after the end of the intervention programme. In both experimental and control groups, girls accumulated less pentathlon hours each week than boys. Results also showed that the regular practice of PA, outside of PE classes, varied between schools, both in control and experimental conditions. At times, the weekly volume of physical activity also varied greatly among students from the same classes. Conclusion: Team pentathlon brought about a greater volume of physical exercise among ten- and eleven-year-old children. Although girls reported a slightly larger raw volume of physical activity per week than boys, they accumulated lower pentathlon hour values due to their choice of less intense activities. Finally, the results also showed that regular PA, outside PE classes, may vary from one school to another or even among students from the same class. (Contains 4 tables, 3 notes and 4 figures.)