Context. In many parts of the world, brown rats have colonised sewer systems and the rat populations in sewers are often thought to be a source of problems with rats on the surface. The management of sewer rat populations is usually performed with little, if any, knowledge of the dynamics and behaviour of these populations.

Aims. The study aimed to uncover the population dynamics, movements and distribution of rats in populations living in sewers.

Methods. We studied rats in a capture–mark–recapture study in two adjacent but separate sewer systems in suburban Copenhagen, Denmark, covering a surface area of 30 Ha. Multiple-capture live traps were placed in manholes in public sewers; animals were marked individually and data were collected every fourth week for a period of almost 3 years.

Key results. In total, 332 individual rats were trapped. Approximately one-third were only caught once, half of these were juveniles. The median time an individual was present in the sewers was 4 months with maxima up to 22 months. Abundance ranged from 2 to 44 animals per sewer system and the dynamics showed no regular fluctuations. Rats were distributed throughout the study area, regardless of sewer dimensions. Juvenile rats were captured more in the smaller and drier sewer sections. Activity areas of rats of both sexes overlapped partially, and observations of bite wounds were rare, suggesting a high degree of tolerance. The average maximal distance covered by rats was ∼200 m for both sexes. Day-to-day movements took place over shorter distances. There were no movements of rats between sewer systems and no sewer rats were observed to be active on the surface. The rat populations seemed to be substructured in smaller groups.

Conclusions. Sewer rat populations vary irregularly in abundance. Pipe dimensions are not important for the presence of rats but breeding seems concentrated in smaller and drier pipes. Most individuals are resident and live in small groups that are active in a limited area only, but still there is a fair group of dispersing rats.

Implications. Sewer rat management should be spread out spatially in a sewer system since most rats move over short areas only. Recolonisation by dispersing rats must be taken into account if sewer systems are connected. It is likely that targeting smaller and drier pipes will affect reproductive success.