In the 3-, 33- and 66-day-old chicken, two muscles, the oxidative slow tonic anterior latissimus dorsi and the glycolytic fast twitch posterior latissimus dorsi were compared by the measurement of muscle fibre diameter and the fraction of total muscle tissue nuclei which were either myonuclei or satellite cell nuclei. Between 3 and 33 days there was a period of rapid growth (more marked in the posterior latissimus dorsi) which coincided with a sharp fall in numerical density of myonuclei and satellite cell nuclei (number per cubic millimetre muscle tissue). The fraction of all nuclei which were satellite cell nuclei declined steadily. The higher levels of myonuclei and satellite cell nuclei in the anterior latissimus dorsi were thought to be a reflection of its oxidative metabolism and the presence of multiple endplates. The volume of sarcoplasm occupied by single myonuclei in anterior and posterior latissimus dorsi muscles was shown to be considerably greater than that occupied by nuclei in other cell systems.