Myelin is a multi-layered membranous lipid insulator surrounding axons that allows the rapid conduction of neuronal impulses. In the central nervous system (CNS), myelin is produced by oligodendrocytes. During development, morphologically immature oligodendrocyte precursor cells (OPCs) arise from neural stem cells before differentiating into myelinating oligodendrocytes shortly after birth. Fyn tyrosine kinase (Fyn) has been shown to play a central role during OPC differentiation, including inducing morphological changes in the cells and initiating the expression of myelin basic protein (MBP), a major structural protein required for the compaction of myelin sheaths. Recently, we have shown that signaling via the gamma chain of immunoglobulin Fc receptors (FcRgamma) induces the Fyn-MBP cascade and promotes the morphological differentiation of OPCs. The protein tyrosine phosphatases that are responsible for the positive regulation of Fyn tyrosine kinase activity during this cascade, however, remained unknown. Here we report that a protein tyrosine phosphatase, CD45, is involved in this process. Fyn co-immunoprecipitated with CD45 from differentiating wild-type OPCs in vitro, while CD45-deficient OPCs failed to differentiate. Additionally, dysmyelination was observed in CD45-deficient mice in vivo. Our findings suggest that CD45 is a key phosphatase involved in OPC differentiation and provide a preliminary explanation for the previously reported CD45 mutations observed in some multiple sclerosis (MS) patients.