A new esterase locus (Es-13) has been identified in Musculus. Strains AEJ/GnRk, LG/J, SJL/J, and SWR/J carry a recessive allele, Es-13b, for a locus possibly involved in the posttranslational modification of a kidney esterase. All other strains observed carried the dominant Es-13a allele. Es-13 was mapped on Chr 9 by recombinant inbred lines and by conventional backcrossing experiments. Backcross data produced the following gene order and map distances: Lap-1 (31.6 +/- 7.5 cM) Es-13 (2.6 +/- 2.6 cM) Mod-1.