This study examined the effectiveness of an intervention to increase levels of moderate-to-vigorous intensity physical activity (MVPA) during girls' physical education lessons. Two Year 7 classes (age 11-12 years) were randomly appointed to control and experimental groups. Both followed the same six-lesson unit of gymnastics with identical lesson objectives. The experimental class teacher included the additional objective of increasing MVPA during each lesson. MVPA was assessed in all six lessons using heart rate (HR) monitoring and systematic observation. After each lesson, students' intrinsic motivation and perceived competence were assessed, and the teachers evaluated whether they had met planned objectives. The experimental group engaged in more MVPA [F[subscript 1, 21] = 8.49, P = 0.008 (HR), t[subscript 8] = -2.35, P = 0.048 (observation)] than the control group and also had most opportunities for skill practice (t[subscript 8] = -2.81, P = 0.023). Intrinsic motivation and perceived competence levels were similar between the groups for each lesson, and teachers reported that lesson objectives were satisfactorily achieved. This intervention succeeded in increasing MVPA without compromising intrinsic motivation, perceived competence or planned lesson objectives. (Contains 4 tables and 1 figure.)