A light and electron microscopic study was initiated on the olfactory tissues of larval winter flounder (Pseudopleuronectes americanus) and haddock (Melanogramus aeglefinus) that had been exposed to 500 ppb of copper (cupric chloride dissolved in seawater) for a period of 18 hrs. Light microscopic examination of the affected tissues demonstrated moderate to severe olfactory lesions characterized by cellular necrosis, extensive vacuolization, and rupture of both receptor and supporting cells along their apical borders. Electron micrographs of minimally damaged tissues suggested that the initial effects of the toxicant resulted in the disruption and dissolution of the membranous organelles (i.e. RER, SER, Golgi system) in a manner comparable to those observed for mercury on the central nervous system of other vertebrates (Cheng, 1977). This study shows that the sensory tissues of larval fish, like those of adult fish, are readily damaged by the presence of excess copper. Disruption of the sensory tissues (i.e. olfaction) may exert its influence on larval fish survival by modifying feeding or other significant behaviors.