Sixty-two Adult Artillery Recruits, 95 Junior Infantry Soldier Recruits and 104 Junior Infantry Leader Recruits were studied before and after 3, 5 and 11 months, respectively, of British Army basic training. Before basic training the mean maximal oxygen uptake predicted from cycle ergometry ([Vdot]O₂max) for Adult Artillery Recruits was 51.6ml(kgmin)⁻¹. It was 58.3ml(kgmin)⁻¹’ for the Junior Infantry Soldier Recruits and 58.0ml(kgmin)⁻¹ for the Junior Infantry Leader Recruits. For Adult Artillery Recruits, after basic training there were statistically significant increases in body weight ( + 2.1%) and pred [Vdot] ₂max ( + 3.6%) but mixed responses for muscular strength, endurance and fatigue. For Junior Infantry Soldier Recruits, there was no significant change in body weight but a significant reduction in pred [Vdot] ₂max (−2.4%) and a trend towards increased isometric muscular strength. For Junior Infantry Leader Recruits, there were significant increases in body weight ( + 4.9%), pred [Vdot] ₂max (+30%), and isometric muscular strength. These results suggest that the intensity and nature of 3 months of basic training for Adult Artillery Recruits was sufficient to improve their aerobic fitness but was not effective in materially improving muscular strength and endurance. The intensity and nature of basic training for Junior Infantry Soldier Recruits over 5 months was effective in increasing muscular strength but resulted in a decrease in aerobic fitness possibly on account of their high initial fitness level. Basic training over 11 months for Junior Infantry Leader Recruits was effective in increasing body weight, aerobic fitness and muscular strength. This may have reflected a change in the intensity and nature of training and in lifestyle after the initial 5 months of basic training, which was undertaken in common with the Junior Infantry Soldier Recruits, or it may be related to normal ageing.