Trichodina domerguei Wallengren, 1897 and Trichodina tenuidens Fauré-Fremiet, 1944 infestations on three-spined stickleback (Gasterosteus aculeatus L., 1758) were studied in the period from January to May 2000 in the Black Sea coasts of Sinop, Turkey. The overall infestation prevalence and mean intensity level were 60.9% and 109.1 ± 21.5 trichodinids per fish, respectively. Statistically significant differences existed for the trichodinid species among sites on the fish, among length classes and between the sexes of fish hosts. While T. tenuidens itself represent a new parasite record, Gasterosteus aculeatus is a new host record for T. domerguei in Turkey. Many structures of the parasites were smaller in March, when the habitat was brackish, compared to May, when waters were fresh.