Since 1991, herpesvirus infections have been reported among larvae and juveniles of various bivalves. Most of the studies focused on detection of viral infections of economically important species. However, the persistence of bivalve herpesviruses in the marine environment is poorly documented. The present study concerns the role of seawater parameters in Ostreid Herpesvirus 1 (OsHV-1) detection by polymerase chain reaction (PCR). Viral DNA extracted from purified particles or virions present in infected oyster larvae were detected by PCR after storage in different media at different temperatures. The lowest detection threshold was found using distilled water or Tris EDTA buffer. In seawater, the threshold was higher. The use of sterile media permitted detection of viral DNA stored over a longer period. Storage temperature also had a significant influence on detection, with lower temperatures promoting DNA detection over a longer period. In summary, water parameters such as temperature influenced detection of OsHV-1 DNA by PCR. However, the PCR technique may also be successfully applied to samples in natural seawater. Indeed, the PCR technique permitted detection of naked viral DNA at 100 ng l -1 in seawater in bioassays.KEY WORDS: Oyster Herpesvirus 1 · Viral DNA · Seawater · Detection · PCR · Temperature Resale or republication not permitted without written consent of the publisherDis Aquat Org 62: [35][36][37][38][39][40][41][42][43][44] 2004 outbreaks of mortality reported among Ostrea edulis and C. gigas spat were associated with detection of a herpes-like virus in France (Comps & Cochennec 1993, Renault et al. 1994a,b, 2000a. Herpes-like virus replication has also been observed in O. angasi adults in Australia (Hine & Thorne 1997) and in larval Triostrea chilensis in New Zealand (Hine 1997, Hine et al. 1998). Herpes-like viruses thus appear ubiquitous in bivalves. They are detected in different parts of the world, at different stages of development in different species. Moreover, herpes-like virus infections in bivalves are often associated with substantial mortalities. These observations highlight the importance of using a range of efficient diagnostic methods in order to assess the causative role of these viruses in bivalve mortalities. Using viral DNA extracted from infected C. gigas larvae (Le Deuff & Renault 1999), different molecular techniques, such as the polymerase chain reaction (PCR), have been developed (Renault & Lipart 1998, Renault et al. 2000b. PCR is used successfully to detect viral DNA during productive infections in oyster larvae and spat (Renault et al. 2000b). An in situ hybridisation technique has also been described (Lipart & Renault 2002). This technique may confirm the presence of herpes-like viruses in histological sections and localise viral DNA in different tissues and organs of infected individuals. Although these molecular techniques are now used to detect herpes-like viruses in marine bivalves when abnormal mortalities occur, no data are available about the dete...