Human skeletal muscle consists mainly of 2 types of fibres denoted type I (slow twitch) and type II (fast twitch). The aim of this investigation was to study skeletal muscle fibre characteristics together with physical performance, physical activity and interest in different physical activities, and the relationships between these variables among students in grade I in high school. Sixty‐nine boys and 47 girls at the age of 16 were studied. The relative proportion of type I fibres (type 1%) in vastus lateralis was approximately 50% for both boys and girls and the interindividual variation was large, more so for boys than for girls. The muscle fibre areas for all fibre types were larger in boys than girls and the relationships between the sizes of the areas of type I, type IIA and type IIB differed between the sexes. The muscle enzymes succinate dehydrogenase, 3‐hydroxyacyl‐CoA‐dehydrogenase and phosphofructokinase were all on average higher in boys than girls; the only significant difference was in succinate dehydrogenase. There was a positive correlation between performance in a 9‐min run and type 1%. for boys but not for girls. Multiple correlation analysis indicated that factors determining the performance in the 9‐min run were maximal oxygen uptake and type I% for boys, and maximal oxygen uptake but not fibre types for girls. The attitude to cross‐country running was positively correlated with the type I% for boys but not for girls. There was a stronger positive correlation between degree of physical activity and type I% for boys than girls. In conclusion, stronger correlations between muscle fibre characteristics and physical performance, physical activity and interest in physical activity were found for boys t