Morphological, chromosomal, distributional, and ecological data are presented for three species of Sigmodon (S. alleni, S. arizonae, and S. mascotensis) from the state of Nayarit, Mexico. The species were collected in all possible pairwise combinations of sympatry, including the first record of such documented for S. arizonae and S. mascotensis. Emphasis is devoted to the discrimination of S. arizonae and S. mascotensis, from each other and from typical S. hispidus, using qualitative features of the skin and skull and morphometric analyses of craniodental measurements. Based on these results and examination of type specimens, additional synonyms of S. mascotensis are identified, with reassignment of two forms, tonalensis Bailey (1902) and obvelatus Russell (1952), currently mistaken as subspecies of S. hispidus. Sigmodon mascotensis emerges as a species distributed from southern Nayarit and Zacatecas to extreme western Chiapas, where it inhabits deciduous or semi-deciduous tropical vegetation having a pronounced dry season. These reallocations and other reidentifications remove any documentation for S. hispidus along the entire Pacific versant of Mexico.
