In his discussion of the pocket gophers of Baja California, Huey (Trans. San Diego Soc. Nat. Hist., 10:245-268, map, August 31, 1945) made no mention of specimens from Ensenada, Baja California, recorded by Bailey (N. Amer. Fauna, 39:58, November 15, 1915) as _Thomomys bottae nigricans_. We have examined the specimens from Ensenada available to Bailey in the U. S. Biological Surveys Collection, Nos. 137724, 139890, and 139891, subadult, immature, and adult, respectively. As compared with _Thomomys bottae sanctidiegi_ from the mouth of the Tiajuana River (No. 126028) and _T. b. nigricans_ (topotypes), the one adult specimen from Ensenada agrees with _T. b. sanctidiegi_ and differs from _T. b. nigricans_ in lighter color, larger and more angular skull, and more inflated braincase. The specimens from Ensenada differ from the adjacent subspecies to the south, _Thomomys bottae proximarinus_ [to judge from Huey's (_op. cit._) characterization of that subspecies] in lighter color, and larger, more robust skull. Accordingly, the specimens from Ensenada are referred to _Thomomys bottae sanctidiegi_. ~Thomomys bottae siccovallis~ Huey Bailey (N. Amer. Fauna, 39:58, November 15, 1915) listed a specimen from Mattomi, Baja California, as _Thomomys bottae nigricans_. When Huey (Trans. San Diego Soc. Nat. Hist., 10:259, August 31, 1945) revived the name _Thomomys_ [_bottae_] _aphrastus_ Elliot, and named (_op. cit._:258) _Thomomys bottae siccovallis_ he made no mention of the specimen, from Mattomi, which, on geographic grounds, would be expected to be _T. b. aphrastus_, _T. b. martirensis_ J. A. Allen, or _T. b. siccovallis_. We have examined an adult male (10832 CNHM), probably the specimen seen by Bailey (_loc. cit._), from Mattomi, and have compared No. 10832 with six topotypes (10813-10816, 10819 and 10820 CNHM) of _T. b. martirensis_, the type and one topotype (10798 CNHM) of _T. b. aphrastus_ and with the original description of _T. b. siccovallis_. The specimen from Mattomi seems to be unique in the large size of the tympanic bullae. The specimen in question differs from _T. b. martirensis_ also in shorter and wider skull, shorter and wider rostrum, and longer and wider molariform teeth. In these features resemblance is shown to the holotype of _T. b. aphrastus_ and even greater resemblance is shown to _T. b. siccovallis_ to which the specimen from Mattomi is referred. ~Thomomys monticola mazama~ Merriam