This is the first study on the ultrastructure of the zoospores of Ichthyodinium chabelardi, a parasitoid of the fish egg and early larval stages. The zoospores were characterized by the cell structure specific for dinoflagellates; particularly, cells contained large trichocysts and the "dinokaryon"-type nucleus. An unusual large electron-transparent zone was the only significant difference from the "classical" cell structure in Dinoflagellata. We did not find cell structures for the penetration to the host cell (microtubular basket, conoid, or secretory organelles such as rhoptries). The data on the fine structure of the zoospores of I. chabelardi agree with the results of molecular phylogeny; this allows us argue that excluding this species from Dinoflagellata and assigning it to Protalveolata was a mistake.