Creating environments in physical education (PE) that foster perceived competence and physical activity during gender-mixed game play lessons is a challenge, especially with adolescent girls. This study is a small experiment in one PE lesson that aimed to increase the perceived competence and in-class physical activity in girls, by applying a single-gender grouping strategy within co-educational classes. A final sample of 216 students (90 girls; 42%) within 13 classes in grades 7-9 (age 11-15) played basketball in mixed-gender and in single-gender teams. The effects on participant's perceived competence and moderate-to-vigorous activity (MVPA) were assessed using questionnaires and heart rate monitors, respectively. Although girls' perceived competence was lower than that of boys, girls' perceived competence increased during single-gender game play. Physical activity levels were high during both mixed-gender and single-gender game play. Playing invasion games (i.e. basketball, handball, soccer) in gender-specific groups could be a useful strategy for PE teachers to implement into their lessons, in order to improve girls' perceived competence during invasion games.