During the 1976-77 brood year, approximately 12 cases of neuroblastoma were observed in a captive group of 100,000 fingerling coho salmon (Oncorhynchus kisutch) reared in a commercial hatchery. The tumors were large, occurring in the skeletal muscle near the dorsal fin causing conspicuous bulging of the overlying integument. Tumors examined from 3 fish each consisted of neuroblasts in trabecular patterns interspersed by glial fibrillar material and linear cavities resembling central neural canals lined by ependyma-like cells. Ganglion-like cells also were apparent morphologically and by special stain. Cancer of the tumor was characterized by an abundance of mitotic figures with occasional abnormal divisions, local invasion of normal tissues, and potentially metastatic tumor cell aggregates in organ vasculature. The etiology of this tumor may have been related to mutagenic-carcinogenic halogenated compounds possibly formed in the hatchery water supply during continuous chlorination of incoming river water.