This study examined gender differences in the physical demands of British Army recruit training in 3 platoons with different gender compositions. Subjects wore heart rate monitors to measure cardiovascular strain and accelerometers to measure physical activity during weeks 1, 6, and 9 of the 12-week program. There was no difference in physical activity between platoons or genders (p > 0.05). In the mixed gender platoon, males operated at a lower cardiovascular strain than females (24 +/- 2 vs. 33 +/- 2% heart rate reserve (HRR), p < 0.001), probably because of their greater aerobic fitness (p < 0.001). Males in the mixed gender platoon experienced lower cardiovascular strain than the male-only platoon (24 +/- 2 vs. 33 +/- 2% HRR, p < 0.001). Females experienced the same degree of cardiovascular strain, irrespective of platoon (33 +/- 2 vs. 33 +/- 3% HRR, p = 0.814). The additional cardiovascular strain experienced by female recruits may increase fatigue and predisposition to overuse musculoskeletal injury.