Introduction: The objective of physical training in military training is to develop physical and psychological preparation in adverse environmental conditions. Objective: Determine the changes in body composition and somatotype product of basic physical training in military training for four weeks in conscript soldiers. Methods: 28 men were evaluated (age: 18.9 ± 0.9 years; height: 1.75 ± 0.1 m; weight: 67 ± 8.7 kg; BMI: 22.5 ± 2.4 and ∑6 folds 70.88 ± 26.57) who entered the period of military training in the Chilean Air Force, Quintero headquarters. Informed consent was given before the study; the indications established in the Declaration of Helsinki were followed. The participants were incorporated into a physical training plan for 4 weeks. The program was carried out in the facilities of a military center. Results: After physical training there was a significant decrease in adipose mass (p<0.01) and an increase in muscle mass (p<0.01). There were no significant differences in the somatotype components (Pre: 3.29-4.78-2.59 vs. Post: 2.73-4.72-2.73; p = 0.24). Conclusion: It is concluded that physical training in the military, during a short period of four weeks, causes rapid and significant changes in the body composition of the subjects that are not observed with such clarity through the somatotype.