Complementary interacting molecules on myelin and axons are required for long-term axon-myelin stability. Their disruption results in axon degeneration, contributing to the pathogenesis of demyelinating diseases. Myelin-associated glycoprotein (MAG), a minor constituent of central and peripheral nervous system myelin, is a member of the Siglec family of sialic acid-binding lectins and binds to gangliosides GD1a and GT1b, prominent molecules on the axon surface. Mice lacking the ganglioside biosynthetic gene Galgt1 fail to express complex gangliosides, including GD1a and GT1b. In the current studies, CNS and PNS histopathology and behavior of Mag-null, Galgt1-null, and double-null mice were compared on the same mouse strain background. When back-crossed to >99% C57BL/6 strain purity, Mag-null mice demonstrated marked CNS, as well as PNS, axon degeneration, in contrast to prior findings using mice of mixed strain background. On the same background, Mag- and Galgt1-null mice exhibited quantitatively and qualitatively similar CNS and PNS axon degeneration and nearly identical decreases in axon diameter and neurofilament spacing. Double-null mice had qualitatively similar changes. Consistent with these findings, Mag- and Galgt1-null mice had similar motor behavioral deficits, with double-null mice only modestly more impaired. Despite their motor deficits, Mag- and Galgt1-null mice demonstrated hyperactivity, with spontaneous locomotor activity significantly above that of wild type mice. These data demonstrate that MAG and complex gangliosides contribute to axon stability in both the CNS and PNS. Similar neuropathological and behavioral deficits in Galgt1-, Mag-, and double-null mice support the hypothesis that MAG binding to gangliosides contributes to long-term axon-myelin stability.