The myxosporidian Thelohanellus nikolskii has been recorded in young of carp in fish-breeding ponds of Moldavia since 1982. Sporogenesis takes place from June to August. Single cysts with mature spores are found in winter and spring. The distribution of cysts in the host's population is of overdispersed character. Haematological investigation has revealed the threshold level of the invasion when there are observed changes in the blood of infected fishes (8 to 10 cysts per 1 g of fish weight). The parasite is most pathogenic during the spore formation. Blood of fishes contains increased contents of younger forms of neutrophils, monocytes and thrombocytes.