seem to be much the same. In the investigation described here I attempted to evaluate various types of habitats on the basis of their carrying capacity at different stages of the annual cycle and in different years. The habitats were studied and described in terms of yield, cover and species composition. The areas upon which live-trapping was done were studied most intensively. These two areas, herein designated as House Field and Quarry Field, were both occupied by voles throughout the period of study. Population density varied considerably, however (Fig. 5). Both of these areas were dominated by _Bromus inermis_, and, in clipped samples taken in June, 1951, this grass constituted 67 per cent of the vegetation on House Field and 54 per cent of the vegetation on Quarry Field. Estimates made at other times in 1950, 1951 and 1952 always confirmed the dominance of smooth brome and approximated the above percentages. Parts of House Field had nearly pure stands of this grass. Those traps set in spots where there was little vegetation other than the dominant grass caught fewer voles than traps set in spots with a more varied cover. _Poa pratensis_ formed an understory over most of the area studied, especially on House Field, and attained local dominance in shaded spots on both fields. The higher basal cover provided by the _Poa_ understory seemed to support a vole population larger than those that occurred in areas lacking the bluegrass. Disturbed situations, such as roadsides, were characterized by the dominance of _Bromus japonicus_. This grass occurred also in low densities over much of the study area among _B. inermis_. Other grasses present included _Triodia flava_, common in House Field, but with only spotty distribution in Quarry Field; _Elymus canadensis_, distributed over both areas in spotty fashion and almost always showing evidence of use by voles and other small mammals; _Aristida oligantha_ and _Bouteloua curtipendula_, both more common on the higher and drier Quarry Field; _Panicum virgatum_, _Setaria_ spp., especially on disturbed areas; and three bluestems, _Andropogon gerardi_, _A. virginicus_ and _A. scoparius_. The bluestems increased noticeably during the study period (even though grasses in general were being replaced by woody plants) and they furnished a preferred habitat for voles because of their high yield of edible foliage and relatively heavy debris which provided shelter.