European beavers, being so called a "keystone species", transform their habitat according to their own needs while also creating favorable living conditions for other animals. The aim of the research was to find out how different species of mammals use beaver dams and the area directly adjacent to the watercourse, in the place where the dam was built, in the autumn and winter period. In the investigated areas the activity of 13 different mammal species were recorded, most often using the beaver dams as river crossing, either on or close to the dam itself. Between September-February the activity of mammals was evenly distributed over the period of each month with a slight increase in November. The study confirmed a great role of the european beaver in increasing the diversity of the local mammal species.