Pacific salmon and steelhead trout in a Californian hatchery were found to be infected with typical PKD. The source of the infection culd not be traced. During November, the PKX organism disappeared from the stocks of coho and chinook salmon but persisted in the steelhead trout where they were observed in the interstitial tissue and tubular epithelium of the kidney. In these convalescent fish, but not in uninfected fish, another protozoan, consistent with myxosporidan trophozoites was seen in the lumen of tubules. The possibility that the two parasites are developmentally related is discussed.