The relation between number of myelin lamellae and axon size in the CNS was examined by electron microscopy of spinal cord white matter fibres in different vertebrate species (cat, rabbit, guinea pig, rat, mouse, frog and perch). The results show that the number of myelin lamellae increases with increasing axon size in a non-linear fashion. Below an axon size of 4--5 micron the relation follows a fairly straight line but above this size rectilinearity is lost. The mouse and the frog differ from the pattern shared by the other animals. In the mouse the lamellar number increases more slowly with axon size and the relation is close to linear. In the frog the number of lamellae increases very slowly with axon size and the relation is markedly curvilinear. Measurements of the myelin repeating period show that in the mammals and the frog the average period of thick sheaths is about 85% of that in thin sheaths, in accordance with previous findings in the cat. In the perch a clearcut difference in this respect between thick and thin sheaths is not found. Calculations of the g-ratio on the basis of the findings indicate that it increases with increasing fibre size. This is most pronounced in the perch and the frog in which the g-ratio for the largest fibres far exceeds the functionally optimal value defined in theoretical analyses on impulse propagation.