Salmonella contamination of North Sea water was detected for the first time in 1988 in Germany during routine examinations of bathing areas. Since then, subsequent isolations along the coast have been reported regularly. To define the source of contamination, strains isolated from seawater and rivers were studied by molecular marker methods. Their properties were compared with those of strains originating from possible sources of contamination such as humans, cattle, and sewage treatment plant water. Plasmid profile analysis of whole bacterial populations and the determination of antibiotic resistance patterns demonstrated, that contamination through the surrounding cattle industry could be excluded. Cattle isolates belonged to a widespread clone of phage type 204c which was multiresistant and exhibited an unique plasmid pattern which was never found in sea water isolates. Outer membrane protein and lipopolysaccharide analysis failed to demonstrate differences among the Salmonella populations and proved in this case insufficient for molecular marker discrimination.