1891. _Oryzomys aquaticus_ J. A. Allen, Bull. Amer. Mus. Nat. Hist., 3:289, June 30, type from Brownsville, Cameron Co., Texas. 1918. _Oryzomys couesi aquaticus_, Goldman, N. Amer. Fauna, 43:39, September 23. 1960. _Oryzomys palustris aquaticus_, Hall, The Southwestern Nat., 5:173, November 1. _Distribution in Tamaulipas._--North part of state, and coastal area south to Tampico. Weights of two males were 80 and 82, and of a female 66 grams. _Oryzomys palustris aquaticus_ differs from _O. p. peragrus_ in having a rich cinnamon, reddish color and the interorbital region constricted to less than 14.7 per cent of the greatest length of the skull. _O. p. peragrus_ is ochraceous and grayish. The least width of its interorbital region is more than 14.5 per cent of the greatest length of the skull. Individuals studied from the Sierra de Tamaulipas are typical _aquaticus_. Of those from Altamira, one has the color as in _aquaticus_, but the color of the other two resembles that of _peragrus_; nevertheless, all of the mentioned specimens are here assigned to _aquaticus_. _Records of occurrence._--Specimens examined, 4: Sierra de Tamaulipas, 10 mi. W, 2 mi. S Piedra, 1200 ft., 1; 6 mi. N, 6 mi. W Altamira, 2; 5 mi. N, 5 mi. W Altamira, 1. Additional records: Camargo (Goldman, 1918:40); Matamoros (_ibid._); near Cd. Tampico (Ingles, 1958:395). =Oryzomys palustris peragrus= Merriam 1901. _Oryzomys mexicanus peragrus_ Merriam, Proc. Washington Acad. Sci., 3:283, July 26, type from Río Verde, San Luis Potosí. 1918. _Oryzomys couesi peragrus_, Goldman, N. Amer. Fauna, 43:39, September 23. 1960. _Oryzomys palustris peragrus_, Hall, The Southwestern Nat., 5:173, November 1. _Distribution in Tamaulipas._--Western part of state, along Sierra Madre Oriental. Two males from Jaumave weighed 62 and 65 and one pregnant female weighed 67 grams. Most records of _O. p. peragrus_ are from places along the Sierra Madre Oriental, but Lawrence (1947:103) recorded a specimen from the Río Corona, which is east of, but not far from the mentioned Sierra. Baker (1951:215) reported two specimens from two different localities labeled with reference to Ciudad Victoria (same specimens reported here) as _O. p. aquaticus_, but pointed out that they tended "toward the darker _O. c. peragrus_." Examination of more material and taking into consideration the relation between the interorbital constriction and the greatest length of skull, cause me here to refer those specimens to _peragrus_. Hooper (1953:8) reported three young specimens from Rancho Pano Ayuctle as of the subspecies _aquaticus_, but study of two adults from the same locality reveals that this locality should be included within the geographic range of _peragrus_.