_Comparisons._--_Perognathus flavescens cockrumi_ averages approximately 12 per cent smaller in linear measurements than the more northern _Perognathus flavescens perniger_ Osgood (from Knox, Stanton and Cumming counties, Nebraska) but color of upper parts is essentially the same. From the more western _Perognathus flavescens flavescens_ Merriam (from Seward, Hamilton and Morton counties, Kansas), _cockrumi_ differs in being darker in all parts of the pelage except on the underparts which are white in both subspecies; the parts of the hairs that are Ochraceous-Buff in _cockrumi_ are Light Ochraceous-Buff in _flavescens_; the back of _cockrumi_ is blackish instead of yellowish. From the more southern _Perognathus flavescens copei_ Rhoads (topotypes examined but not at hand as I write), _cockrumi_ differs in duller more blackish (less bright and less reddish) upper parts. From _Perognathus merriami gilvus_, of more southern distribution, the new subspecies differs in much smaller tympanic bullae and wider rostrum. _Measurements._--The type, a male (35331/47596 U.S.B.S., from Cairo, Kansas, showing some wear on P4), and another male (60165 K. U., from Barber Co., Kansas, showing much wear on P4) measure, respectively: Total length, 114, 120, 124; tail, 51, 55, 58; hind foot, 17, 17, 18; occipitonasal length, ----, 21.0, 21.6; condylobasal length (condyles to anterior end of premaxillae), 18.5, 18.6, 19.3; frontonasal length, ----, 14.1, 14.3; mastoidal breadth, 10.5, 11.2, 11.2; length of bulla, 6.8, 7.1, 6.8; interorbital breadth, 4.7, 4.8, 5.1; alveolar length of upper molariform tooth-row, 3.1, 3.1, 3.0; interparietal breadth, 4.3. 4.6, 4.7. _Remarks._--The subspecific name _cockrumi_ is proposed in recognition of Dr. E. Lendell Cockrum's important contribution to our knowledge of the mammals of Kansas. Dr. W. Frank Blair recently suggested to me that the two specimens examined by him from Kansas (the one from Ellsworth County and the one here designated as holotype) should not be referred to _Perognathus flavescens copei_ Rhoads, as Cockrum (Univ. Kansas Publ., Mus. Nat. Hist., 7:146, August 25, 1952) had done, because _copei_ is paler, instead of darker, than _P. f. flavescens_. It was Dr. Blair's suggestion which lead me to realize that the subspecies in south-central Kansas lacked a name.