Muskrats (Ondatra zibethicus) are associated with marshlands throughout North America. Their impact on marsh vegetation is well documented. In recent years, research pertaining to marsh ecosystems has emphasized the role of interspersion of marsh vegetation to higher faunal diversity and productivity. Muskrats can provide a natural control of aquatic emergents if managed properly. Experimentation at Fish Springs National Wildlife Refuge, Utah, has attempted to ascertain whether vegetation-utilization index could be used for determining muskrat population levels and to evaluate the impact of muskrats on their primary food resource, Olney's bulrush (Scirpus americanus). Three known populations of muskrats were maintained within wire-mesh enclosures 0.4-0.9 ha in size. Variable length transects were used to estimate the number of grazed stems of Olney's bulrush present in an enclosure for a 5-day period. Estimated number of grazed stems was divided by muskrat-days to give an estimate of the number of grazed stems muskrat-1 day-1. A consistant relation between the known muskrat population and the number of stems grazed muskrat-1 day-1 (23 .0) was found, indicating that a valid index could be formulated using vegetation utilization data. Assessment of plant species utilization was analyzed from dietary composition obtained from stomach contents. Olney's bulrush was the most utilized food resource. Phragmites (Phragmites australis) was the most utilized resource in the absence of bulrush. The potential effects of muskrat grazing was determined using exclosures in homogeneous stands of the above vegetation. A 4x4 randomized block design with varying levels of simulated grazing was employed for monthly replications duringthe growing season for 2 years. Effect of repeated grazing was found to be highly significant for clipping rates within and between months for both years. Vegetative yield was measured as mean dry weight (g) per stem per plot. Significant differences in yield were noted among the grazing rates. Repeated grazing of emergent vegetation by muskrats can have a marked effect on the plants' ability to grow.