A new liver-specific rat carboxylesterase isozyme (EC 3.1.1.1) designated esterase-18 (ES-18) is described. Genetic variation of ES-18 was examined in 93 inbred strains and substrains and a structural locus Es-18 was suggested, coding for either the presence (Es-18a) or the absence (Es-18b) of the isozyme. Linkage studies involving two backcross series revealed that Es-18 resides in cluster 2 of LGV. No recombination between Es-18 and other cluster 2 loci was found in 19 lines of two RI strain sets or in the backcross series.