Since Sin Nombre virus, a hantavirus, was first discovered in the U.S. in 1993, the Centers for Disease Control and Prevention have sponsored longitudinal studies on the rodent reservoir host, the deer mouse (Peromyscus maniculatus). These studies have demonstrated a qualitative correlation among mouse population dynamics and risk of Hantavirus Pulmonary Syndrome in humans. Therefore understanding mouse population dynamics is the first step in creating a predictive model for hantavirus outbreaks. Using an extensive dataset from Montana, which includes mark-recapture histories of over 2000 mice, we determined mouse population demographic parameters, such as juvenile and adult survivorship, recruitment, and maturation rates, and created a periodic stage structured matrix population model to describe the dynamics. Analyses suggest both density dependent and independent factors are operating in this system. Weak seasonality and considerable inter-annual variation in the demographic parameters exist in these deer mouse populations. Elasticity analyses highlight important parameters in determining the population dynamics. Understanding the population dynamics of hantavirus reservoir hosts may allow public health officials to predict increased human risk and effectively target prevention strategies.