Urosporids parasitize wide range of animals (echinoderms, annelids and ascidians) inhabiting coelom, intestine etc. Translocation within host causes changes in morphology and motility mode together with a structural modification of cortical zone. Particularly, gregarines of the family Urosporidae exhibit considerable morphological plasticity. This family comprises representatives inhabiting various host biotopes and showing different types of motility: gliding movement (Urospora travisiae), metaboly (Urospora ovalis) or peristalsis (Pterospora floridiensis), as well as immotile species (Urospora chiridotae, Lithocystis spp. and Gonospora spp.). Trophozoites of U. ovalis are spindle-shaped and lack any signs of cell polarity and show metabolic activity. TEM confirms that trophozoites are covered by a trimembrane pellicle. It forms numerous longitudinal epicytic folds that extend from one to another cell pole. In the apical part of these folds, two or three apical arcs can be poorly observed between the plasma