We tested the hypothesis that habitat complexity explains alpha diversity of nonvolant small mammals along an elevational gradient in southern Mexico. During October-November 2003, we conducted fieldwork on the Pacific slope of El Triunfo Biosphere Reserve. Small mammal trapping was conducted using standardized techniques (trap lines and pitfalls) along an elevational gradient between 500 and 2100 m elevation. Habitat assessment as indicated by vegetation complexity and diversity was conducted at each site (N = 12). Nine species and 148 individuals were captured in 8400 trap-nights. Results indicate that non volant small mammal diversity increases with habitat complexity. In addition, our study shows that the spatial pattern of diversity cannot be attributed to spatial autocorrelation.