Metchnikovellids (Microsporidia: Metchnikovellida) are poorly studied hyperparasitic micro-sporidia that live in gregarines inhabiting the intestines of marine invertebrates, mostly poly-chaetes. Our recent studies showed that the diversity of the metchnikovellids might be signifi-cantly higher than previously thought, even within a single host. Four species of metchnikovellids were found in the gregarines inhabiting the gut of the polychaete Pygospio elegans from littoral populations of the White and Barents Seas: the eugregarine Polyrhabdina pygospionis is the host for Metchnikovella incurvata and M. spiralis, while the archigregarine Selenidium pygospionis is the host for M. dogieli and M. dobrovolskiji. The most common species in the White Sea is M. in-curvata, while M. dobrovolskiji prevails in the Barents Sea. The gregarines within a single worm could be infected with different metchnikovellid species. However, co-infection of one and the same gregarine with several species of metchnikovellids has never been observed. The difference in prevalence and intensity of metchnikovellid invasion apparently depends on the features of the life cycle and on the development strategies of individual species.