The forms differentiated within the genus Peromyscus may roughly be classified according to degree of distinction as local races, subspecies or geographic races, species, species groups, and subgenera. Different subgenera and different species groups, so far as they have been tested in this genus, are completely intersterile. Within the truei species group two included species are partially intersterile. In two other Peromyscus species groups the included species are potentially interfertile, but in nature are separated partly by geographical and partly by psychological (sexual) barriers, so that no interbreeding occurs. Within any given species of Peromyscus all the subspecies, so far as they have been tested, are potentially interfertile, but in nature some adjacent subspecies are separated by geographical, by ecological or by sexual barriers. Sexual isolation seems in this genus to be of especial importance in speciation.