A procedure for concentrating small round-structured viruses (SRSVs) (Norwalk-like viruses) from water and other environmental materials is described. Primers based on the helicase region of the SRSV genome were confirmed as specific by reaction with typed specimens, and used to detect virus in concentrates of unseeded and seeded samples. Virus was detected in estuarine recreational water polluted by untreated sewage, although not in seawater samples taken some distance from outfall discharges. It was also detected in river water downstream of a sewage treatment plant. Virus could be detected in all matrices when they were seeded with a positive stool extract, including sewage seeded with as little as 2 microl stool extract, thus confirming the suitability of the method for environmental monitoring.