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An outbreak of the Microsporidian parasite Thelohania contejeani is reported in a population of Austropotamobius pallipes in Northumberland, U.K. Since less than 10% of all crayfish caught were infected it is concluded that T. contejeani has no serious effect on the population studied. The mode of transmission of the parasite from host to host is discussed.The host–parasite relationship and life-cycle of T. contejeani were studied using histological and electron microscope techniques. Spores were observed in some detail, and diplocarya, sporont and pansporoblast stages were identified within the general spore mass that invades host muscle tissue. Using serial sections the sporoblast ‘diplocaryon’ was shown to be a single nucleus with a deep cleft. T. contejeani was found to occur mainly in striated muscle, although it was found in cell bodies in the central nervous system.