Examination of cottontail rabbits from Arizona in the Biological Surveys Collection and the United States National Museum indicates that _Sylvilagus audubonii_ can be distinguished from _Sylvilagus nuttallii_ and _Sylvilagus floridanus_ by the larger (more inflated) tympanic bullae. Topotypes of _Sylvilagus nuttallii pinetis_ and other specimens from Alpine, Mt. Thomas, Springerville, the Prieto Plateau at 9000 feet on the south end of the Blue Range, and the Tunitcha Mountains are characterized by a posteriorly pointed supraoccipital shield and a long, wide space between the braincase and the posterior extension of the supraorbital process. The cottontails with equally small tympanic bullae from more western and more southern localities are referable to _Sylvilagus floridanus holzneri_ on the basis of a posteriorly truncate or emarginate supraoccipital shield and a narrower and shorter space (usually a "foramen") between the braincase and the posterior extension of the supraorbital process. In _S. f. holzneri_ the posterior end of the posterior process fuses with the braincase whereas the posterior end of this process in Arizonan specimens of _S. n. pinetis_ merely lies against the braincase or projects free of it. In specimens from Arizona the difference in shape of the posterior border of the supraoccipital shield and the difference in size of the space between the braincase and the posterior extension of the supraorbital process are the only differences of taxonomic worth found by us. Many other features of the skull, of color of pelage, and of size of external parts all fell within the range of individual variation of a series of specimens from one locality. Specimens from the following localities in Arizona are referable to _Sylvilagus floridanus holzneri_ (Mearns). Hualpai Mts., Nos. 117461, 117462, 117488, 117490, 117495, 227735, and 227832; Ft. Whipple, No. 214157; Prescott, No. 34667/46752; Mayer, No. 247495; Reynolds Creek Ranger Station, Sierra Ancha Mts., Gila Co., No. 247734; Fish Creek, Tonto National Forest, 2000 ft., No. 212833; north base Mt. Turnbull, 4500 ft., No. 214339; Ash Creek, 6100 ft., Graham Mts., No. 204363; Pinery Canyon, 7500 ft., Chiricahua Mts., No. 247953; Thomas Canon, 2 mi. E Baboquivari Mts., No. 244420; Pine Springs, 15 mi. south of Colorado Canon, No. 2425 Amer. Mus. Nat. Hist. On December 4, 1950, we removed the skull of No. 2425 to more certainly ascertain the identity of the individual.