Human hereditary neurodegenerative diseases are genetically and mechanistically very heterogeneous and so are spinal muscular atrophies and cerebellar ataxias in the mouse, despite the common phenomenon of neuronal death. In this species, a number of mutations impair spermiogenesis in addition to neuron survival. Among these, the wobbler mutation on proximal chromosome 11 of the mouse leads to motoneuron degeneration in brain stem and spinal cord and to a defect of spermiogenesis. Chimeric mice of the type wr?/wr? → +/+ were produced, and their allelic status at the wr locus was determined by PCR diagnosis of a closely linked marker. Two overt chimeras, one female (XX → XX) and one male (XY → XY) were identified as wr/wr → +/+ and analyzed with respect to their pathological phenotype. Although there was patchy astrogliosis in the spinal cords of both chimeras, their motor performances were overtly normal and muscles were without signs of denervation. The male's testes revealed a mosaic pattern of normal and pathological spermatids. As no progeny was derived from wr spermatids, the spermatocytes appear as a primary target of the wr mutation in testis. Our results argue against a humoral mechanism of the wobbler disease and indicate a cell‐autonomous action of the wr gene both in testis and in spinal cord. Dev Dyn 209:286–295, 1997. © 1997 Wiley‐Liss, Inc.