Proliferative kidney disease (PKD) is a serious infection of wild and farmed salmonids, affecting mainly the kidney and spleen but becoming systemic in most susceptible fish hosts. This report deals with the transmission of Tetracapsula bryosalmonae Canning, Curry, Feist, Longshaw & Okamura 1999 from naturally infected bryozoans Fredericella sultana Blumenbach 1779 to naive rainbow trout Oncorhynchus mykiss Walbaum 1792, thereby confirming the recent conclusion based on partial 18S rDNA sequence data that bryozoans are hosts of the myxozoan parasite T. bryosalmonae (formerly PKX organism) that causes the disease. Parasite transmission using T. bryosalmonae spores was successful by short-term exposure to disrupted bryozoans known to contain T. bryosalmonae spores and T bryosalmonae sacs liberated from the bryozoans, and by long-term cohabitation with infected bryozoan colonies. Infection was confirmed by examination of kidney imprints, detection of the parasite in stained tissue sections, PCR using T. bryosalmonae-specific primers, and comparison of amplified 18S rDNA sequences from the bryozoans and experimentally infected fish. Transmission was not apparent, nor was PKD induced, in fish challenged by intraperitoneal injection of spores isolated from F. sultana.