The Sierra de Tamaulipas is placed in the Tamaulipan Biotic Province because the fauna, especially of non-flying mammals, is closely related to that of the rest of the Province. Nevertheless, many mammals found in this Sierra are tropical in relationship. This is especially true of the bats. Therefore, most of the tropical bats that occur in Tamaulipas occur in the Veracruzian Biotic Province and in the Sierra de Tamaulipas. Potosian Biotic Province This Province occupies all of the Sierra Madre Oriental and, therefore, the southwestern part of the state. The vegetation in general is Pine-oak Forest, in which the most common trees are _Abies religiosa_, _Pinus flexilis_, _P. patula_, _P. montezumae_, _P. teocote_, _Populus tremuloides_, _Juniperus flaccida_, _Quercus arizonica_, _Q. clivicola_ and _Q. polymorpha_. In his study of plants of the Gómez Farías area, Martin (1958) recorded several different types of vegetation, which in part can be placed in the Potosian Biotic Province, especially those types that occur to the northwest of the Cloud Forest. In addition to the Cloud Forest, Martin recognized Humid Pine-oak Forest, Dry Oak-pine Forest, Chaparral, Thorn Forest and Scrub, and Thorn Desert. The only mammal endemic to the Potosian Province in Tamaulipas is _Cryptotis pergracilis pueblensis_. Other mammals that occur mainly in this Province are: _Sorex saussurei_; _Notiosorex crawfordi_; _Glaucomys volans herreranus_; _Cratogeomys castanops planifrons_; _Perognathus nelsoni_; _Liomys irroratus alleni_; _Reithrodontomys fulvescens griseoflavus_; _Microtus mexicanus subsimus_; _Ursus americanus eremicus_; _Conepatus leuconotus texensis_; and _Odocoileus hemionus_. The fauna of this Province is a mixture of elements with tropical affinities on the east side of the Sierra Madre and with those of the Mexican Plateau on the west side. Chihuahuan Biotic Province This Province occurs in Tamaulipas only in a small portion of the Central Plateau physiographic region and occupies the southwesternmost part of the state. The vegetation is of two types: Desert or Mesquite-grassland. The last is like that described for the Tamaulipan Biotic Province. In the Desert type the dominant plants are the cactus, _Opuntia leptocaulis_, and yuccas, _Yucca filifera_ and _Y. potosina_. Subdominants are mariola, guayule, _Agave lechugilla_, _A. stricta_ or _Larrea divaricata_. Along stream banks mesquite, _Prosopis juliflora_, can be found. No endemic mammals of the Chihuahuan Province are known in Tamaulipas. Mammals that occur principally in this Province are: _Dipodomys merriami atronasus_; _D. ordii durranti_; _Peromyscus melanophrys consobrinus_; _P. difficilis petricola_; _Onychomys torridus subrufus_; and _Neotoma albigula subsolana_.