As of October 1996, there were 444 loci mapped to Chr 18 in the mouse (Table 1); 110 identified as genes or gene-related sequences and 334 identified as anonymous DNA sequences. Since last year's report, 67 new loci were added; 21 identified as genes and 47 identified as DNA segments. In addition to these 444 loci, 19 chromosomal aberrations involving Chr 18 have been reported and also are included in Table 1. The composite genetic map of Chr 18 has been anchored to the centromere by reference to the loci Rnr18 and Hc18 in two different multipoint crosses (1701, 3749). Average recombination frequencies between common reference loci from several wellcharacterized crosses were used to construct a framework map upon which all other loci were placed. Map positions in Table 1 are given as genetic distances in centiMorgans (cM) from the centromere; however, because recombination frequencies may vary among different linkage crosses, these map positions should be interpreted as relative rather than absolute. Gene order appears to be conserved among the mouse strains used in linkage studies; therefore, when locus order could be established from multipoint crosses, this information was given priority over estimated interlocus recombination distances for assigning locus positions in Table 1.