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• Crassostrea gigas from the inner zone of Bahía Blanca Estuary, Argentina were examined. • Macroscopic lesions in mantle tissues were found in eight specimens. • Histological analysis revealed abnormality of mantle epithelial cells and connective tissues. • Histology and PCR revealed 70% to be positive for presence of OsHV-1 DNA. • The nucleotide sequence was 99% identical to Ostreid herpesvirus 1 strains from France and Ireland. Ostreid herpesvirus 1 (OsHV-1) is a DNA virus of the genus Ostreavirus (Malacoherpesviridae family, Herpesvirales order). Worldwide, OsHV-1 and its microvariants have been associated with increased mortality of Pacific oysters, Crassostrea gigas. Adult asymptomatic oysters also have shown a high prevalence of viral infection. As a consequence, surveillance is needed to better describe OsHV-1 diversity, pathogenicity, clinical signs, and geographical distribution. We examined Crassostrea gigas sampled in October 2017 from the inner zone of the Bahía Blanca Estuary, Argentina, and found that 8 of 30 specimens (26.7%) presented macroscopic lesions in mantle tissues. Histological analysis revealed abnormal presentation of mantle epithelial cells and connective tissues. Conventional and real-time PCR conducted on the oyster samples revealed 70% to be positive for presence of OsHV-1 DNA. The nucleotide sequence of the amplicon obtained from one sample using the primer pair IA1/IA2 (targeting ORF 42/43) was 99% identical to OsHV-1 reference as well as µVar strains B and A (KY271630, KY242785.1), sequenced from France and Ireland. This finding represents the first detection of OsHV-1 DNA in a wild population of C. gigas in Argentina in association with gross mantle lesions. [ABSTRACT FROM AUTHOR]