PTPalpha interacts with F3/contactin to form a membrane-spanning co-receptor complex to transduce extracellular signals to Fyn tyrosine kinase. As both F3 and Fyn regulate myelination, we investigated a role for PTPalpha in this process. Here, we report that both oligodendrocytes and neurons express PTPalpha that evenly distributes along myelinated axons of the spinal cord. The ablation of PTPalpha in vivo leads to early formation of transverse bands that are mainly constituted by F3 and Caspr along the axoglial interface. Notably, PTPalpha deficiency facilitates abnormal myelination and pronouncedly increases the number of non-landed oligodendrocyte loops at shortened paranodes in the spinal cord. Small axons, which are normally less myelinated, have thick myelin sheaths in the spinal cord of PTPalpha-null animals. Thus, PTPalpha may be involved in the formation of axoglial junctions and ensheathment in small axons during myelination of the spinal cord.