The inactivation rates in tbe aquatic environment of two fish pathogens, infectious pancieatic necrosis virus (IPNV) and infectious haematopoietic necrosis virus (IHN V), were compared with that of poliovirus type 1 as a representative of the human enterovirus group. The survival studies were performed using untreated fresh, estuarine and sea water samples beld at 15 and 20"C. The results indicated that tbe salmonid viruses survived longer than poliovirus in the saline waters, whereas in fresb water poliovirus was the most stable of the three viruses. IPN virus proved to be most stable in estuarine water at 15"C, whereas the survival of IHN virus was favoured in fresb water. We also observed that at 20^C the inactivation rate for each virus was independent of salt concentration in estuarine and sea water. Although temperature exhibited a marked effect on virus stability in fresh water, the salmonid viruses presented similar survival patterns at both temperatures in sea water. In general tbe period of greatest viral inactivation correlated with higher bacterial numbers in the waters.