In captivity, both species constructed nests that were indistinguishable to me, when the mice were given cotton, kapok, or pieces of burlap as building material. The cotton or kapok was used directly, but the burlap was shredded into a fine mass of fluffy fibers. The burlap seemed to me to be the best building material, for it maintained its shape best. Both species constructed nests that resembled inverted bowls. Solitary mice naturally built smaller nests than those built by females with young. The entrance to the closed nests varied; often the female would bolt through the side of the nest where there was no opening. Sometimes the mice would exit and enter through the top of the nest. In some cases it appeared that the entire nest was closed; probably the occupant had closed the entrance. Such a closed nest would have the advantage of greatly moderating the microenvironment within the nest, and would allow the animal within to remain comfortable with a minimum expenditure of energy. The larger nests found in the trapping grid resembled those built by captives. Nests built of grasses were always larger than those built of juniper bark. Juniper bark is as easily worked into nests as are grasses, in my judgment. Therefore, difficulty of construction of nests from this material probably does not account for the smaller size of the nests composed of bark. I think the difference in insulating characteristics between the two materials probably accounts for the difference in size of the nests. REPRODUCTION In Mesa Verde, _Peromyscus_ reproduces from April through September. Reproduction is greatly reduced in the autumn, and most females complete reproduction before October. Ten of the 20 females of _P. maniculatus_, taken in May, contained embryos; five others were lactating. Lactating and pregnant females were collected on May 5, 1962, indicating that reproduction in some females began in early April. In September, 15 of 34 females were pregnant or lactating, whereas in October only two out of 15 females of _P. maniculatus_ were reproducing. Only one female of _P. maniculatus_ was found to contain embryos in October. This large adult was taken on October 3, 1963, and had six embryos, each five millimeters long. She probably would have produced a litter later in October, and would have been nursing into November. A report of October breeding in north-central Colorado described nine of 23 females of _P. maniculatus_ as being in a reproductive state; seven were lactating and one was pregnant between October 26 and 31, 1952 (Beidleman, 1954:118). In the Museum of Natural History, the University of Kansas, there are 35 females of _P. maniculatus_ more than 144 millimeters in total length taken from Mesa Verde in November, 1957 (Anderson, 1961:53). None of these contained embryos, and no pregnant females have been taken from the park in November.