The morphology and physiology (temperature-growth relationships) of seventeen isolates of Saprolegnia from fish hatcheries in Japan were compared. A linear relationship between the number of hairs per bundle against bundle length, on secondary zoospore cysts, was found. One isolate of S. parasitica, a particularly strong pathogen, from silver salmon, had a secondary zoospore cyst ornamentation consisting of separate bundles of long, short and intermediate length hairs. This feature was also seen to a lesser extent in other isolates, including a strong pathogen from brown trout in the U.K.
Using the Japanese isolates, temperature-growth relationships at 30 ??C distinguished isolates of S. parasitica, from rainbow trout, from those from other fish. A mutant of one of the rainbow trout isolates showed differences in growth rates compared with the parent isolate, the possible ecological significance of which is discussed.