R. zweifeli, sp. nov. of the R. tarahumarae group, is named from southwestern Mexico. This species occurs from southern Jalisco through parts of Colima, Michoacan, Mexico (state), Morelos, Guerrero and Oaxaca. R. zweifeli is electrophoretically distinct from the other members of the R. tarahumarae group that occur in western Mexico (R. pustulosa and R. tarahumarae) and it is distinct in several morphometric and pattern characteristics from all the described species in the group.