Background: Physical education should promote an active and healthy lifestyle with an emphasis on students' preparation for lifelong physical activity. "Healthy People 2010" recommends that physical education is offered on a daily basis and that pupils engage in physical activities of moderate to vigorous intensity (MVPA) during at least 50% of lesson time. Purpose: The present study aimed at investigating the potential of modified game forms to promote sex equity in 13-year-old pupils' physical activity engagement during coeducational lessons. A second purpose was to compare levels of MVPA between single-sex and coeducational classes with and without ability grouping. Methods: To provide evidence for transfer of results between countries, the study was conducted in Belgium and Portugal. The sample consisted of 221 students out of 12 Belgian and 12 Portuguese classes, including 104 Portuguese and 117 Belgian pupils, of whom 113 were boys and 108 were girls. Two classes were all-girls, two all-boys and eight coeducational. One identical invasion game lesson (korfball) was taught using modified game forms as a core part of the lesson. Variables such as class size and sex composition, court measurements, and the orientation of the physical education teacher were controlled for. Polar Heart Rate Monitors were used to register heart rates in order to measure physical activity levels. Time spent in MVPA was calculated using the following formula: MVPA = (number of heart-rate readings higher than 145 beats per minute x 15 seconds)/60 seconds. Results: Results showed that the percentage of MVPA was significantly higher among girls (69.9%) when compared to boys (56.8%) and this was the case for all lesson contexts (coeducational and single sex). For pupils participating in the single-sex classes, 55.0% of the heart-rate recordings exceeded the threshold value for MVPA; for coeducational classes with and without ability grouping, percentages were significantly higher with values of 68.1% and 65.5% respectively. Conclusion: It can be concluded that using modified game forms with simplified rules has the potential as a coeducational instrument to produce MVPA levels that correspond with physical activity guidelines in both sexes, independently from lesson context. The findings of this study offer a promising means of, in particular, stimulating girls' physical activity levels during physical education. (Contains 2 figures and 2 tables.)