Invasive plant species have widespread effects on the ecosystems they inhabit. Extensive research has been done on the economic and ecological impact of invasive species in relation to native plant species, but little is known about the effect of these species on native fauna. My study examined the impact of an invasive plant species Elaeagnus umbellata (autumn olive) on small mammals populations in southern Illinois. Elaeagnus umbellata is a native shrub of southeast Asia and was introduced to the United States in the 1830s. It is now the fifth most abundant invasive plant species in the state of Illinois. To determine the impact of this invasive shrub on small mammal populations, Sherman traps were set up in six separate trapping webs, three in upland forests with heavy E. umbellata cover and three in upland forests with little to no E. umbellata cover (control). Trapping spanned three seasons from March-August 2013. Through the use of capture and release methods, individuals were measured, weighed, and marked for possible recapture. Beginning in season 3, Cuterebra fontinella (bot fly) abundance in small mammals was also assessed. Differences in relative abundance of small mammal populations were determined using a repeated measures ANOVA with a Tukey test. T-tests were used to determine any significant difference among small mammals between forest types. Very little difference was found among the measured variables between the invaded and control forest sites. Prevalance of C. fontinella was also similar, but the resulting percentage of infection was much higher when compared to other studies in the literature. These results indicate there is little effect of E. umbellata on small mammal populations, although the long-term impacts of this invasive species may need further research.