Three strains of a new Phlebotomus fever group virus were isolated from pack rats (Neotoma micropus) collected in south Texas during 1973--1974; the name Rio Grande was proposed for this virus. The virus is pH 3.0 labile, sensitive to the action of sodium deoxycholate and heat (56 degrees C) labile. The results of a serosurvey indicated that pack rats are probably the principal vertebrate host for Rio Grande virus and that year-round transmission of the virus may occur. Because no isolations of this virus were made from hematophagous insects, the vector, if any, remains undetermined.