Agamococcidians (Agamococcidiorida Levine, 1979) represent an enigmatic group of Apicomplexa. The life cycle of these parasites is characterised by presence of oocysts with sporocysts (similar to coccidian), sporozoites and trophozoites, and by absence of merogony and gamogony. This group combines two families Rhytidocystidae Levine, 1979, parasites of polychaetes, and Gemmocystidae Upton & Peters, 1986, parasites of stony corals. The phylogenetic position of these organisms is still unclear. Previous phylogenetic studies brought agamococcidians together with gregarines and cryptosporidians (Leander, Ramey, 2006; Rueckert, Leander, 2009; Kristmundsson et al., 2011; Cavalier-Smith, 2014). In contrast, morphological data (oocysts with sporocysts and nonmotile trophozoites located within host epithelial tissues) indicate a relationship of agamococcidians with coccidians. We isolated two putative new species of Rhytidocystis from polychaetes Pectinaria hyperborea and Ophelia limacina collected in the Keret Archip