Slow and fast isoenzymes of myosin coexist in all the fibres of a fast-twitch mammalian muscle during early development. They later become segregated into different populations of fibres. Slow myosin is most abundant when the speed of contraction of the muscle is slow and the fibres are multiply innervated; its synthesis in the majority of the fibres seems to be 'switched off' when the speed of contraction increases and the fibres become innervated by single motoneurones.