PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON 114(4):972-987. 200L A new species of tufted-tailed rat, genus Eliurus (Muridae: Nesomyinae), from western Madagascar, with notes on the distribution of E. myoxinus Michael D. Carleton, Steven M. Goodman, and Daniel Rakotondravony (MDC) Division of Mammals, Department of Systematic Biology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560-0108, U.S.A.; (SMG) Field Museum of Natural History, Roosevelt Road at Lake Shore Drive, Chicago, Illinois 60605, U.S.A. and WWF, B.P 738, Antananarivo (101), Madagascar; (DR) Departement de Biologic Animale, Universite d' Antananarivo, B.P. 906, Antananarivo (101), Madagascar Abstract. — A new species of Eliurus, E. antsingy (Muridae: Nesomyinae), is described from western Madagascar, only the second one of the genus so far known from this vast and biologically underexplored region of the island. The distribution of E. myoxinus, the other species known from western Madagascar, is also amplified on the basis of new collections that have been made over the past decade as interest in small mammals of this region is rekindling. Resume. — Une nouvelle espece d' Eliurus, E. antsingy (Muridae: Nesomyi-nae), est decrite pour la region Quest de Madagascar. Ce n'est que la deuxieme espece du genre connu pour cette vaste region encore biologiquement sous-exploree de Madagascar. La distribution de E. myoxinus, 1' autre espece connue pour I'ouest de Madagascar, est aussi etendue grace aux nouvelles collectes qui ont ete faites au cours de la derniere decennie car I'interet porte aux petits mammiferes est rallume. Among Madagascar's nine endemic ro-dent genera, Eliurus, the so-called tufted-tailed rats, is the most speciose with nine currently recognized species (Carleton 1994, Carleton & Goodman 1998). Eight of these nine species inhabit portions of the great humid forest biome (sensu Humbert 1955) of eastern Madagascar, occurring within both lowland and montane rainforest associations (for example, Carleton 1994, Goodman & Carleton 1996, 1998; Good-man et al. 1999). Only one, E. myoxinus, the type species of the genus, is known to occupy Madagascar's dryer western and southern landscapes, such as dry deciduous forest and xerophilous scrub formations (Carleton 1994, Goodman et al. 1999). Al-though some of the disparity in species richness between east and west probably re-flects actually lower biodiversity, some may be plausibly attributed to the historical in-sufficiency of biological survey of small mammals in Madagascar's drier environ-ments (see Carleton & Schmidt 1990). For example, recent research and collecting in western forests have uncovered four new species of Microcebus or mouse-lemurs and resurrected two others from synonymy (Zimmerman et al. 1998, Rasoloarison et al. 2000). This revision and discovery bring to seven the number of Microcebus species now recognized from a region that was only a few years ago thought to hold one. In this paper, we describe a second spe-cies of Eliurus from western Madagascar. In addition, we report new localities that significantly amplify the known geographic distribution of E. myoxinus and provide western records of two forms apparently re-lated to eastern species.