ABSTRACT: We evaluated differences in productive traits and disease susceptibility among Ostrea edulis stocks. We produced 4 to 5 families from each of 4 oyster populations (Irish, Greek and 2 Galician) in a hatchery. Spat corresponding to 19 different families were transferred to a raft in the Ría de Arousa (Galicia, Spain) for grow-out. Samples of each family were histologically processed every month for 2 yr. One of the pathological conditions disclosed by histological examination was characterised by the occurrence of numerous abnormal cells throughout the connective tissue of various organs, showing hypertrophied nuclei with marginated chromatin and a characteristic large intranuclear acidophilic inclusion. Ultrastructural examination showed that the abnormal cells contained herpesvirus-like particles. In situ hybridisation assay using a DNA probe specific for Ostreid herpesvirus 1 (OsHV-1) confirmed that the abnormal cells were infected by OsHV-1 or a closely related herpesvirus. All cases of this pathological condition, except one, were detected during the first year of grow-out; thus it was mostly restricted to juvenile stages. The disease was detected in oysters of each origin but it was not found in all families of each origin, thus suggesting significant parental influence in the susceptibility to this disease or significant influence of the infective status of the parents on the infection of the progeny (vertical transmission). This pathological condition was likely responsible for oyster mortality to some extent during the first year of grow-out.