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A light and electron microscopic study was conducted on dorsal fin tissues adjacent to acute fin erosion lesions in winter flounder from 2 polluted sites (New York Bight region and New Haven Harbor) on the northeast Atlantic Coast. The objective of this work was to evaluate these minimally affected, lesion-associated tissues which may precede the acute or severe stages of the disease. The following 4 types of pathological conditions were found in the epidermis of diseased fish from the 2 polluted sites: (1) epithelial cell hyperplasia; (2) mucous cell hyperplasia and hypertrophy; (3) spongiosis; and (4) focal necrosis. The latter 2 types of lesions have not been previously reported for fin erosion in this species. Changes in the dermis associated with these lesions included fibrosis, abnormal distribution of melanocytes, hyperemia and sclerosis of blood vessels, and hemorrhage. The possibility that hypoxia may play a role in the observed pathology is considered.