Adverse effects from the medication of fish diseases with sulfonamides have been reported by several workers. A decreased growth rate (Gutsell and Snieszko 1949, Snieszko and Wood 1955), the production of drug resistant strains of pathogens (Snieszko 1952), sterility and kidney damage (Wood et al., 1955), and mortalities (Johnson and Brice 1953) have been described. Recently, increased mortalities and tissue damage were observed in young silver salmon after a single day's treatment with sulfamethazine. Subsequently an attempt to reproduce the conditions experimentally was made. The purpose of this paper is to describe the pathology of acute sulfamethazine toxicity and to discuss certain factors of possible importance in the etiology of losses among fish treated with sulfonamides.