Abstract: Trichodinids are a group of parasitic ciliates that are common and harmful in aquaculture. From August 2017 to July 2018, trichodinid infections in 8 species of farmed fish in Daya Bay were investigated. All measurements are presented in μm and follow the uniform specific characteristic system proposed by Lom (1958). The denticles were described in detail in accordance with the method proposed by Van AS and Basson (1989). A total of five species of trichodinids (Trichodina jadranica Raabe, 1958; T. lepsii Lom, 1962; T. guliae Asmat, 2006; Trichodinella lomi Xu, Song & Warren, 1999; Paratrichodina oblique Lom, 1963) were detected from five species of fish (Trachinotus ovatus, Lutjanus stellatus, Epinephelus lanceolatus, Sciaenops ocellatus and Hulong grouper); of which, Trachinotus ovatus was infected with five species of trichodinids, and Hulong grouper, Lutjanus stellatus, Sciaenops ocellatus and Epinephelus lanceolatus were infected with four, three, two and one species of trichodinids respectively. The annual infection data showed that the highest prevalence of these five species was Trachinotus ovatus, and the lowest prevalence was the two trichodinid species on Sciaenops ocellatu. Trichodina jadranica, T. guliae and Paratrichodina oblique have the highest mean intensity of infection in Trachinotus ovatus, Trichodina lepsii and Trichodinella lomi in Hulong grouper. The seasonal dynamics of the total population of parasitic trichodinids showed that the prevalence and the mean intensity in winter were generally low, but the peak season of infection varies with the species; among them, the mean intensity of Paratrichodina oblique was higher than the other four species except winter, and the peak of infection appears in autumn. The seasonal dynamics of the five trichodinid species of Trachinotus ovatus are similar to that of the total population.