IN adult human and mammalian skeletal muscle there are two types of fibre. One is rich in oxidative enzymes and poor in phosphorylase and usually of smaller diameter, while the other group is rich in phosphorylase and poor in the oxidative enzymes1. In two cases of benign congenital hypotonia2 and in a case of central-core disease3 it was noted that all the fibres had a uniform enzyme activity.