The deltoid and lateral vastus muscles of 113 subjects aged from 1 week to 20 years who had suddenly died from accidents were studied at autopsy 8-48 h after death (mean 22 h). All muscles studied gave a positive histochemical reaction for ATPase after alkaline preincubation and for NADH-TR; the fact that the dehydrogenase was still active indicated that the material was reasonably well preserved. The muscles of children shortly after birth contained about 40% type 1 fibres; the percentage of these fibres increased to about 60% within the first 2 postnatal years and then remained constant. This indicates that the normal type 2 to type 1 transformation during development is not completed at birth. The mean diameter of the muscle fibres shortly after birth was 10-12 micron, and at age 15-20 years it was 40-60 micron. The standard deviation of the diameters within a sample never exceeded 20% of the mean value. The 5-fold increase in diameter corresponded to an increase in cross-sectional area by a factor of 25. The largest increase was observed in type 2 fibres of males (31-fold) which at age 15-20 years tended to be thicker than type 1 fibres while in females of that age type 2 fibres tended to be of smaller diameter than type 1 fibres. Before the age of 15, there was no difference between muscles from males and females, and type 1 fibres were usually thicker than type 2 fibres.