The present paper reports results of TEM study of a metchikovellid microsporidium earlier described as Vivieria spiralis Rotari et Paskerova 2007, re-isolated from gregarines Polyrhabdina sp. inhabiting guts of polychaetes Pygospio elegans sampled at the White Sea silt littoral zone. Free sporogony (FS) occurred in the life cycle of the microsporidium alongside with sac-bound sporogony (SB). Free spores were of typical metchnikovellidean structure, uninucleate, oblong, measured on sections 1.3-1.9 x 2.0-3.2 µm and resided in a sporophorous vacuole. Life cycle included pre-sporogonial stages represented by dikaryotic cells and 4-nucleate cells with coupled nuclei. A multinucleate sporogonial plasmodium of FS split in numerous (>10) sporoblasts. In BS segregation of sporoblasts occurred within thick-walled cysts by internal budding. Spore sacs of this microsporidium measured in average 11.6 x 4.7 µm, were enclosed in external spiral carcass composed of a dense cord embracing the sac. These oval spore sacs conta