Health state of the Japanese scallop, Mizuhopecten yessoensis, cultured in the Experimental Marine Farm in Minonosok Bay (Peter the Great Bay, Sea of Japan) was investigated. The wide spectrum of histopathological changes has been identified in the internal organs and tissues of scallops: prokaryotic infection with prevalence 100%, destruction of digestive epithelium and other changes in digestive system, infiltration of organs by hemocytes and granulocytome formation. The most affected by prokaryotic infection organs are labial palps, lips, esophagus, intestine and gills. Several of the observed alterations seem to be related to prokaryotic infection.