Between June 1998 and July 1999, 3 Alfaro's rice rats (Handleyomys alfaroi) and 3 Bolivar rice rats (Transandinomys bolivaris) from the Área de Conservación Guanacaste, Costa Rica, were examined for parasites. Adult nematodes representing 4 taxa were collected: Physaloptera calnuensis, Stilestrongylus hidalguensis, and Syphacia sp. (from H. alfaroi) and in T. bolivaris. This report represents the first record for P. calnuensis and S. hidalguensis in Costa Rica; furthermore, this is the first time these helminths have been documented infecting H. alfaroi and T. bolivaris. Based on this study and in the compilation of information related to the nematodes described in wild Costa Rican mammals, we determined that 68 helminth taxa have been registered so far (35 named species and 33 taxa) parasitizing 29 species of mammals.