Studied specimens of the blue bream, Abramis ballerus (L.) originated from Dąbie Lake-a component of the Odra River estuary (northwestern Poland). The gills of the host fish yielded a number of adult specimens and different developmental stages of Trichodina kupermani Arthur et Lom, 1984. Among the developmental stages there were post-cleavage and post-conjugation individuals. There were distinct morphological differences observed between these two developmental forms. They both differed also form the adult specimens.