Many studies worldwide have emphasized the need to map the potential of commercial ports to function as receiver and donor areas for nonindigenous species (NIS). In this study, the macroinvertebrate composition of hard and soft sea bottoms, hydrographical conditions, and traffic connections of four Finnish coastal harbours (Naantali, Koverhar, Porvoo, Hamina) were investigated and compared with 'natural' reference areas lacking anthropogenic constructions. The harbours hosted macroinvertebrate communities fairly similar to those found in adjacent reference areas, but sediment community diversity was slightly higher in harbour basins relative to in surrounding areas. A total of eight NIS were registered in the study. With regard to hydrography, NIS that tolerate low salinities, low temperatures and eutrophic conditions are most likely to survive local conditions and hence pose a potential risk for larger areas of the Baltic Sea. A ship traffic analysis revealed that the most important donor and recipient areas of ballast water to and from the study harbours are coastal harbours of other parts of the Baltic Sea and of the North Sea. Some suggestions are made for future harbour surveys along the Finnish coast.