A field survey was carried out to study the occurrence and distribution of viruses causing diseases of major impact in fish farming, namely viral haemorrhagic septicaemia (VHS), infectious haematopoietic necrosis (IHN) and infectious pancreatic necrosis (IPN) in farmed and wild fish in Switzerland. The presence of VHS virus (VHSV), IHN virus (IHNV) and IPN virus (IPNV) in the tissue samples was tested by virus isolation in cell cultures, and subsequent virus identification by immunofluorescence. The sera were screened for anti-VHSV antibodies (VHSV-AB) using a serum plaque neutralization test with complement addition. These data were then compared with results of a similar survey performed in 1984/85, and with data from routine diagnostic work completed at the Centre for Fish and Wildlife Health (FIWI) of the University of Bern from 1978 to 2001. Sampling sites included private and government fish farms as well as natural habitats from all major river catchments in Switzerland. In 2000/01, 522 tissue samples and 1910 sera were collected from 3400 fish. In 1984/85 1239 tissue samples and 694 sera were collected from 1628 fish. During the last 24 years of routine diagnostics at the FIWI, 1776 tissue samples were examined for presence of viruses. The results of the tissue analysis from the surveys in 1984/85 and 2000/01 showed low numbers of sites with virus-positive fish (five VHSV, three IPNV and three VHSV, one IPNV, respectively) in Swiss fish farms and rivers. The sites with virus-positive fish were located throughout the country. The decline in virus-positive cases observed between the two surveys agrees with data from the routine diagnostic work of the FIWI which show a decrease in total virus isolations from approximately 35 cases per year in the late 1970s, to approximately 10 cases per year during the last 10 years. However, in 1984/85 8.3% (58 of 694 serum samples) and in 2000/01 6.3% (121 of 1910 serum samples) proved to be positive for VHSV-AB. The 58 positive samples in 1984/85 originated from 40 of 175 sites (23%) and the 121 positive samples in 2000/01 were from 84 of 217 (29%) sites. These results are indicative of a wider distribution of VHSV than expected from the results of the virus isolations.