A lactating, pregnant female (KU 81833) of _P. b. attwateri_, containing three embryos, was obtained on December 24, 1959, and shows that this subspecies breeds in winter. Accumulated records for the subspecies indicates year-round breeding (see Cockrum, 1952:181). Another female obtained on March 27, 1960, was probably lactating. Pregnant females of _P. b. cansensis_ (KU 84892, 84895, and 84890) were obtained from the type locality on April 1-2, 1961, containing 3, 4, and 5 embryos respectively. This indicates, perhaps, increased breeding in spring; five was the highest number of embryos found in brush mice in Kansas. _Population Studies_ In the period of my study the populations of brush mice became smaller, perhaps owing to the severe winter of 1959-1960. In Cowley County, _P. leucopus_ is now abundant and _P. boylii_ rare where in December of 1959, the opposite was true. It is also possible, of course, that trapping has depleted the populations. Conclusions 1. A new subspecies of brush mouse is named and described from southern Kansas. 2. The new subspecies has smaller eyes and a shorter tail and may be more primitive than _P. b. attwateri_. 3. No significant sexual dimorphism was noted in _P. boylii_. 4. In Kansas, _P. b. attwateri_ is known only from a single locality; _P. b. cansensis_ is known from only two localities, both in Kansas. 5. The cliff-dwelling habit of _P. boylii_ probably isolates populations from one another. 6. The grasslands constitute a barrier for the brush mouse. 7. In Kansas, _P. b. cansensis_ probably is an older population than _P. b. attwateri_. 8. In Kansas the brush mouse is confined to systems of cliffs that are wooded and that are at least 40 feet in height. 9. The brush mouse may be confined to cliffs in part by activities of other rodents. 10. The brush mouse commonly associates with the pack rat. 11. Laelapid mites have been found on specimens of _P. b. attwateri_. 12. Larval ixodid ticks were found on specimens of _P. b. cansensis_. 13. Acorns seem to be the chief food of the brush mouse; insects and seeds are also commonly eaten. 14. The brush mouse is adapted for climbing and probably for a partly subterranean life. 15. _P. b. attwateri_ breeds in winter, as well as in other parts of the year. 16. _P. b. cansensis_ is known to breed in early April. 17. The highest number of embryos obtained from a brush mouse in Kansas is five. Acknowledgments I am indebted to Prof. E. Raymond Hall and to Mr. J. Knox Jones, Jr., for suggestions and editorial assistance. Prof. R. H. Camin identified the ticks and mites recorded herein. Mr. A. Metcalf, Mrs. C. F. Long, and Mr. D. L. Long helped with the field studies and in other ways. Literature Cited BLACK, J. D.