Developmental changes of muscle fibre composition in the various heads of the elbow, knee, and ankle extensors have been studied in three genera of cercopithecid monkeys. In order to circumvent the technical hindrances of usual histoenzymological procedures (fresh muscles need to be frozen at once at -80 °C), immunofluorescence methods were used and technical adjustments were successfully carried out to make the study of formaldehyde-preserved muscles possible. Clear responses to antibodies against adult fast myosin in newborn macaques demonstrated that, at birth, adult myosins have already replaced the fetal isoforms, thus providing a reliable marker for the study of postnatal evolution of the muscle fibre composition. For each one of the three joints, from birth to adulthood, the percentage of slow, fatigue-resistant fibres increases only in that head of the extensor muscle groups which is specialized in maintaining posture by counteracting gravity (the ‘postural’ head). Hence, the question is raised of the relationships between such cytological evolution, developmental changes in postural behaviour, and body weight incr