A new autosomal recessive coat color mutant in the Mongolian gerbil (Meriones unguiculatus) is described: chinchilla medium (symbol c(chm)). The mutant has typical acromelanistic features similar to those of several acromelanistic c locus mutants of other species of mammals. Previously a more severe form of acromelanism (c(h)c(h)) has been described in the Mongolian gerbil. The new allele shows to be allelic with this form. On a nonagouti background compound heterozygotes (aac(chm)c(h)) show an intermediate phenotype that is very similar to that of the Siamese mouse (Mus musculus) and rat (Rattus norvegicus). Homozygotes (aac(chm)c(chm)) display a very dark acromelanistic phenotype reminiscent of that of the sable rabbit (Oryctolagus cuniculus). The gray phenotype (gg) in the Mongolian gerbil resembles the albino locus phenotype chinchilla (c(ch)c(ch)) in mice. We show that the new mutant is not allelic with gray. Fertility and viability of the new mutant are within normal range.