The muskrat (Ondatra zibethicus) is a wetland mammal whose disturbance activities include grazing, burrowing and lodge construction. We evaluated the effects of these disturbances on plant biomass, species richness and diversity, stem density and potential net nitrogen mineralization and nitrification rates in a freshwater tidal marsh on the Hudson River in New York. We hypothesized that muskrats increase floristic richness and diversity by decreasing the biomass of narrowleaf cattail (Typha angustifolia) and that muskrats increase potential net nitrogen mineralization and nitrification rates through aeration and reduced plant uptake because of herbivory. Because muskrats commonly build lodges on or close to creek banks, we separated the disturbance effects of muskrats from the disturbance effects of the creek bank by sampling quadrats along transects placed perpendicular to creek banks at lodge sites. Muskrats decreased biomass, particularly of cattail, but had no measurable effect on stem count, species richness or species diversity. Muskrats increased potential net nitrogen mineralization and nitrification rates; however, this effect was limited to active sites. Creek bank disturbance increased stem count but had no effect on the other variables. Although muskrats did not significantly affect floristic diversity in this study, their disturbance activities did influence soil nitrogen dynamics, which is an important component of wetland function.