A method was proposed for studying population density, microhabitat preference, daily activity and seasonal activity in small rodents. In this method, artificial tunnels (PVC pipes) imitating a natural system of burrows were applied. Their use by rodents was recorded by means of the electronic counters provided with a photoelectric cell. The study was conducted in an alder swamp 110 years old, located in the Kampinos National Park (52°25’N, 20°53’E) during 2003–2005. In this area, the bank vole Clethrionomys glareolus (Schreber, 1780) accounted for 90–99% of the captured rodents. The use of the tunnels by rodents (the number of passages) did not depend on their length within a range of 1–8 m applied in the experiment (P = 0.22). The tunnels were used by day and night, at the highest rate in the evening and at night. The use of tunnels increased when a bait was exposed in the vicinity (P = 0.001). It was positively correlated with population density (estimated with the CMR technique), and varied from season to season (in spring, summer, and autumn). The formula: density = 0.1717 + 0.0304 × mean number of passages per day, enabled the estimation of population density based on the number of passages through the tunnels. It has been found that a single tunnel was typically used by 5 individuals (mean 4.6 individuals, SD = 1.8). The location of tunnels had a significant effect on their use. Tunnels connecting fallen logs or bases of alders (hummocks) were more frequented than those leading to shrubby areas (P <0.000) or to microhabitats covered with herbaceous vegetation (P <0.001). This method enabled a multisided analysis of the behaviour of rodents at a minimum interference in the life of animals. Thanks to the application of electronic counters of passages, it was possible to obtain easily a large number of data. It is proposed to mark rodents with electronic transponders in the future studies to identify the individuals using artificial tunnels. This method could replace the methods used so far in the studies of small rodents, requiring trapping (CMR) or radio-telemetry.