The somatic metaphase chromosome constitutions of 20 Peromjscus species (188 specimens) were described and compared. Peromyscus (Ochrotomys) nuttalli, which has been considered by many mammalogists as a member of a separate genus, Ochroiomys, has a diploid number of 52. All other 19 species possess a diploid number of 48. The lowest number of total chromosome arms is 56 (P. crinitus and P. boylei) and the highest is 96 (P. eremicus and P. collatus). Since the number of centromeres remains the same, Robertsonian fusion is not found in the genus Peromyscus. The X chromosome, with one doubtful exception, is always a large, biarmed element, but the Y chromosome is extremely variable in size as well as in centromeric position. Intraspecific polymorphism is extensive in many species, and in some cases, e.g., P. truei, cytological data strongly suggest that there may be more than one species under one name. The survey indicates that it may be extremely profitable to use Peromyscus cytology to analyze species relationships, population structure, population dynamics and other evolutionary processes. Phylogenetic relationships among the species thus far analyzed are discussed.