1. The relationship between the wet weight of superficial red muscle and the wet weight of the remaining trunk musculature was found to have the form y = 0·036x+0·339. The average weight of superficial red muscle accounted for 7·4% of the myotomal mass.
2. A technique is described which allows quantitative studies to be made on glycogen utilization in these muscles for short periods of swimming. A total of 120 Crucian carp (Carassius carassius L.) were studied at six different swimming speeds ranging from 2·0-4·2 bodylengths/s.
3. The log10 of the total glycogen utilization of the superficial red muscle was found to be linearly related to swimming speed.
4. Statistically significant glycogen utilization from the remaining myotomal musculature only occurred at speeds in excess of around 3 bodylengths/s. At these speeds the superficial red muscle only accounted for 15-20% of the total glycogen mobilized by the swimming musculature.
5. The significance of these results is discussed in relation to the division of labour between the myotomal muscles of fish.