Abstract Masseter, temporal and biceps brachii muscles were histochemically evaluated for fibre type differentiation in 19 aborted fetuses (gestational ages 15–23 weeks), in 2 premature infants of 34 and 36 weeks gestation and in 2 fullterm stillborn infants. There was no convincing evidence of fibre type differentiation in the masticatory muscles before 22 weeks' gestation while a fairly distinct differentiation was seen in biceps brachii specimens at 16 weeks' gestation with ATPase at pH 4.35. Around 22 weeks two types of fibre were found in all specimens with ATPase at pH 4.35. With regular ATPase (pH 9.4) there was a relatively weak difference in staining between different types of fibre in the masticatory muscles. In biceps brachii Type I and Type II were demonstrated. Neonatally 3 levels of staining were seen in biceps brachii with regular ATPase (pH 9.4); while ATPase at pH 4.35 and 4.6 demonstrated the subtypes IIB and IIC. In the masticatory muscles only 2 levels were recognized with regular ATPase and all type II fibres had the characteristics of Type IIC as seen with ATPase at pH 4.35.