In 16 women and 66 men, aged 14 to 36 years, 10 sedentary and 72 athletes, the histochemical characteristics of the vastus muscle fibres have been studied. Muscle biopsies were processed histochemically using the myofibrillar ATPase method, and were classified according to gender, sport activity and type of exercise. The average diameter of muscle fibres was larger in men than in women and in trained individuals than in sedentary ones. The largest percentage of Type I fibres was found in long distance runners; the smallest in those performing karate and triple jump. The largest percentage of Type IIA fibres was found in swimmers and the smallest in footballers. The largest percentages of Type IIB and IIC fibres were found in footballers. The largest average diameters of Type I and Type II fibres were found in swimmers and in long distance athletes respectively. Type I and Type II fibres were dominant in the aerobic group and the anaerobic one, respectively; the percentage of Type IIC fibres was smaller in the anaerobic group.