Mass mortality among pond cultured red drum (Sciaenops ocellatus) was observed in Hong Kong. Affected fish were lethargic and lost their appetite but no lesions on the body surface were apparent. Patches of white to brownish cottony growth on the gills of affected fish were observed and microscopic examination revealed mats of hyaline mycelia with mature zoosporangia and oogonia which were identified as Saprolegnia diclina. During induced sporulation, production of primary and secondary zoospores, oogonia, and antheridia were observed. A physiological study of the growth and sporulation of a representative isolate determined its optimum growth requirements. The isolate can grow from pH 4 to 10, in distilled water, at salinities of 5–30‰, and temperatures of 4–30°. Maximum growth was observed at pH 5 and 8–10, at salinities of 5–10‰, and 25–30°. Production of zoosporangia only occurred in distilled water, 5 and 10‰ salinities, with zoospores released in distilled water and 5‰ salinity. Zoospore release was also observed from 4 to 30° with greater abundance at 25 and 30°, while oogonia and antheridia were produced in distilled water and from 5 to 30‰ salinities and at 20–30°.