Wild rodents (n=248) were trapped in two ecologically distinct sites at El Cielo Biosphere Reserve in the state of Tamaulipas, Mexico, during the summer of 2003. Samples from 199 individuals were tested for Hantavirus antibodies by an indirect enzyme-linked immunosorbent assay (ELISA). Hantavirus antibodies to recombinant Sin Nombre virus nucleocapsid protein were found in seven rodents (3.5%) of a single species, Peromyscus levipes. Antibody-positive rodents were found only in the Cloud Forest site, which had lower rodent species diversity than the Tropical Subdecidous Forest site. Although the identity of the virus in P. levipes remains to be determined, our study provides further evidence that Hantavirus antibody–positive individuals are prevalent in the rodent fauna of Mexico. This is the first survey for Hantavirus antibodies in the rodent fauna of Tamaulipas and the first report of P. levipes as a potential host for a Hantavirus.