The 129 mouse strain has become of increasing interest to neurobiologists due to its importance in gene targeting studies. However it has been pointed out that 129 mice suffer from a number of neuroanatomical idiosyncrasies that may make them less attractive as animal models in neurobiology. Here we show that 129 mice also differ from other commonly used strains in possessing large numbers of unmyelinated axons in their lumbar motor roots. By contrast in all other strains of mice (C57BL/6, C3H, Swiss-Webster) that we studied the axons in the L5 roots are all myelinated. Additionally we show that 129 mice have smaller myelinated axons than other mouse strains and perform poorly in the rotorod test. These characteristics must be kept in mind in studies of mutant mice that are frequently performed on a mixed genetic background containing a129 contribution.