The occurrence of Trichodina domerguei on the round goby, Neogobius melanostomus, was studied from October 1999 through September 2000. Overall prevalence and mean intensity levels were recorded as 77.9% and 156 ± 43.7 Trichodina per infested fish, respectively. Average host length was 71.1 ± 2.2 mm. Statistically significant differences of parasite prevalence and intensity in relation to season and length of fish were observed. Seasonal variations were also observed in the dimensions of T. domerguei. Neogobius melanostomus is a new host recorded for T. domerguei. This is the first ecological study of T. domerguei conducted in an alternately brackish and freshwater environment.