A large flock of male and female white turkeys (Meleagris gallopavo) was reared for 22 weeks after hatching. At two weeks intervals small groups of birds were removed from the flock and killed. Samples of sartorius muscles were taken from the bird nearest the mean weight for its group. Transverse frozen sections were tested for ATPase activity by calcium method at pH 9.4. Fibres were separated into two categories (strong-ATPase and weak-ATPase) and their mean minimum diameters were measured with a micrometer scale in the microscope eyepiece. Mean minimum diameters of different fibre types were compared using the logarithmic form of Huxley's allometric growth equation. Weak-ATPase fibres grew at a relatively faster rate than strong-ATPase fibres in both males and females (k = 1.12, P less than 0.01 and k = 1.14, P less than 0.01 respectively). Transition of muscle fibres from one histochemical type to another was not detected, so that differences in the rate of increase of mean diameters of different fibre types were attributed to allometric radial growth. The possibility that transitional fibres might make some contribution cannot, however, be totally ignored.