Scanning electron-microscopic (SEM) studies of the egg and oncomiracidium of Zeuxapta seriolae showed the following: the egg has a single filament at the pole opposite the operculum, its surface is smooth except for small rounded protuberances on the shell, and the fracture line between operculum and shell of the main part of the egg has numerous irregular transverse elevations. The non-ciliated surface of the oncomiracidium has many irregular folds, and ciliated cells bear many short microvilli between the cilia. Cilia taper towards the tips. Transmission electron-microscopic (TEM) studies of the cilia and epidermis of the oncomiracidium showed that both ciliated cells and non-ciliated epidermis are nucleated, in contrast to findings on Monopisthocotylea. Ciliated cells are separated by lateral cell membranes connected by septate junctions, there are surface microvilli, many cristate mitochondria, and cilia have single horizontal (anteriorly directed) hollow rootlets. In the ciliary tips, the number of microtubules gradually decreases, and apical parts of cilia have a diameter of about 1/4 of that of the more basal parts. The non-ciliated epidermis also contains mitochondria, as well as deep evaginations of the basal lamina. Phylogenetic implications are discussed. Presence of nuclei in the larval epidermis cannot be due to delayed shedding of nuclei as a result of neotenic development as suggested for polystomes, because Zeuxapta does not have neotenic development. Further studies of additional species must show whether tapering ciliary tips and a reduction in the number of axonemal microtubules are a synapomorphy of all Monogenea, and whether presence of false vertical rootlets of epidermal cilia is a synapomorphy of the Monopisthocotylea.