Mouse embryos, homozygous for mutations at the Splotch locus, are afflicted with spina bifida and disturbances of neural-crest-derived tissues, e.g. spinal ganglia and pigment cells. The development of Schwann cells is affected in homozygotes to a varying degree along the rostrocaudal axis. In cervical motoric roots, nerves are associated with apparently normal Schwann cells. At the thoracic level, nerve-associated cells become more scarce and resemble the surrounding mesenchymal cells. They are not enveloped by a basal lamina and frequently show wide gaps between neighbouring cells. Lumbar motoric roots are mostly devoid of any associated cells. The Splotch mutant embryo is proposed to be a new animal model for the study of peripheral nerve ensheathment. The implications for Schwann-cell-mediated axon guidance are discussed.