We isolated the species Polychaos annulatum (Penard, 1902) Smirnov et Goodkov, 1998 from a freshwater habitat in the surrounding of Saint-Petersburg. The previous re-isolation of this species took place in 1998; at that time the studies of its lightmicroscopic morphology were limited with the phase contrast optics, and the electronmicroscopic data were obtained using the traditional glutaraldehyde fixation, preceded with prefixation and followed by postfixation with osmium tetroxide. In the present paper, we provide modern DIC images of P. annulatum. Using the fixation protocol that includes a mixture of the glutaraldehyde and paraformaldehyde we were able to obtain better fixation quality for this species. We provide some novel details of its locomotive morphology, nuclear morphology, and ultrastructure. The present finding evidence that P. annulatum is a widely distributed species that could be isolated from a variety of freshwater habitats.