A juniper-pinyon-mountain mahogany association extends from the area of Mug House to Rock Springs, on Wetherill Mesa. On that part of the ridge just above Mug House, the understory is predominantly _Cercocarpos montanus_ (mountain mahogany), but northward toward Rock Springs the understory changes to _Fendlera rupicola_, _Amelanchier utahensis_, _Cercocarpos_, and _Purshia tridentata_. The ground cover is essentially the same as that in the pinyon-juniper-muttongrass association described previously. Four traplines were run from July 31 to August 2, 1962, and from August 13 to 15, 1963. These lines ran northwest-southeast, starting 1,000 feet southeast of, and ending 3,000 feet northwest of, Mug House. The lines traversed elevations of 7,225 to 7,325 feet. Individuals of _P. maniculatus_ and _P. truei_ were caught here (Table 1). Deer and rabbits inhabit the trapping area. Bobcats have been seen, by myself and by others, near Rock Springs. Lizards of the genera _Cnemidophorus_ and _Sceloporus_, as well as gopher snakes were seen in this area. _Juniper--Pinyon--Bitterbrush_ Three pairs of traplines were run from August 7-9, 1962, in a juniper-pinyon-bitterbrush stand on the southern end of Wetherill Mesa, starting 200 yards southwest of Double House (Fig. 1). The forest on the southern end of the mesas consists of widely-spaced trees, which reflect the low amounts of precipitation at these lower elevations. Juniper trees are more numerous than pinyons, and both species are stunted in comparison to trees farther north on the mesa. _Purshia tridentata_ (bitterbrush) is the understory codominant. _Artemisia nova_ (black sagebrush) is present and grasses are the most abundant plants in the ground cover. Herbaceous species in the sparse ground cover include the following: _Opuntia polyacantha_ _Solidago petradoria_ _Lathyrus pauciflorus_ _Penstemon linarioides_ _Lupinus caudatus_ _Yucca baccata_ _Phlox hoodii_ Only _P. maniculatus_ was caught in this stand; all mice were caught in the first night of trapping. Five areas were selected for trapping in the summers of 1963 or 1964, in order to test hypotheses concerning habitat preferences of each of the species of _Peromyscus_. Four of these areas appeared to be ideal habitat for one species, but not for the other. The fifth area was expected to produce both species of _Peromyscus_. Each of these areas is discussed below. _One Mile Southeast of Park's Entrance_ A small stand of _Artemisia tridentata_, occurring one mile southeast of the entrance to the park, is bordered to the north and northeast by a grassy meadow, discussed in the following account. Kangaroo rats have been reported in this general area, and I wanted to determine whether _P. maniculatus_ and _Dipodomys_ occurred together there. Fifty trap nights in this sagebrush, on June 20, 1963, yielded only _P. maniculatus_ (Table 1). _Meadow, One-Quarter Mile Southeast of Park's Entrance_