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Proliferative kidney disease (PKD) in salmonid fishes is caused by a poorly understood protozoan, the vegetative stages of which are known as PKX cells. The organism was initially believed to be an amoeba (phylum Sarcomastigophora) by Plehn (1924) and later by Ghittino et al. (1977) and Ferguson and Needham (1978). Similarities between PKX and oyster pathogens of the genus Marteilia led Seagrave et al. (1980) to suspect that the parasite was instead a haplosporidan (phylum Acetospora). More recent studies by Hedrick et al. (1984) and Kent and Hedrick (1985a,b) indicated that the PKX cells are pre sporogonic forms of a myxosporidan (phylum Myxozoa). Kent and Hedrick (1986) showed that polar capsule formation occurs but that the spores fail to fully mature in the salmonid host. Although PKD has been recognized as a major problem among farm-reared trout in Europe for many years, it was not detected in North America until the early 1980's. Of major concern has been the appearance of the disease among Pacific salmon in California, Washington, and British Columbia. Whether the increase in prevalence reflects a spread of the disease or an improvement in its recognition (Hedrick et al. 1985a) has yet to be determined.