A method to detect low levels of infectious salmon anemia virus (ISAV) in environmental samples has been developed. The method is based on concentrating the viruses by tangential flow filtration and ultracentrifugation prior to amplification of the extracted viral RNA by nested reverse transcription polymerase chain reaction (RT-PCR). In the current investigation, seawater samples from salmon holdings were used for ISAV identification. ISAV was detected in seawater samples from a salmon holding site and from a vessel transporting salmon in 2 consecutive trials of the methodology. When known concentrations of ISAV were added to 21 of sea water, 5.5 viruses ml(-1) (corresponding to a tissue culture titer of TCID50 1.6 x 10(-2) ml(-1)) were detected using this method.