To date, Denman Island disease has been found in 10 localities in British Columbia, Canada. Affected Crassostrea gigas have green focal lesions on the surface of the body, mantle and palps. To confirm the etiology, examination of stained tissue imprints for Mikrocytos mackini was more sensitive and rapid than preparing and screening histological sections. The seasonal occurrence of M. mackini in the field severely curtailed the amount of work that could be conducted. A method based on successive centrifugations on sucrose gradients (utilized for the purification of related protozoa such as Bonamia spp. and Marteilia spp.), was developed to isolate M. mackini from infected tissues. The resulting large numbers of microcells were injected into healthy oysters, thus, allowing the propagation of the parasite in the laboratory year round. The results of 10 experiments indicate that 61.5% to 100% of the oysters became infected with M. mackini and some oysters were heavily infected within 3 to 6 weeks after the inoculation. The techniques of injecting microcells was used to examine the host specificity of M. mackini. Preliminary results suggest that the eastern oyster (Crassostrea virginica) and the flat oyster (Ostrea edulis), are more sensitive to M. mackini than C. gigas. At the end of the 11 week experiment, the prevalence of infection was 100%, 92% and 55% for C. virginica, O. edulis and C. gigas respectively, with the intensity of infection much higher in the first two species. These results have to be confirmed by field studies, but they emphasize the potential impact that this disease could have on oyster production world wide if precautions are not taken during the movements of oyster stocks.