High rates of unexplained mortalities (up to 70%) are anticipated in the cultured juvenile sea bass (Dicentrarchus labrax) in farms in the eastern Black Sea at least for the last 8 years. Diplectanum aequans, often blamed for the observed mortalities, is the only prevalent parasite impacting the health of sea bass in the brackish water of the Black Sea. To investigate the seasonal fluctuation of D. aequans prevalence, mean intensities and the potential effects of D. aequans on the fitness of sea bass, eight cages from one farm (Farm A) and four cages at another distinct site (Farm B) were surveyed monthly from May to October of 2008. Six hundred and sixty-one juvenile sea bass from Farm A and 236 from Farm B were individually examined for the presence and intensity of the parasite. The prevalence of D. aequans in cultured sea bass (96.3 ± 11.11; Mean ± SD) ranged from 66.7% to 100% in June, the beginning of 6-month growing season. The mean intensity was 6.08 ± 2.19, significantly higher than that (2.74 ± 0.87; P < 0.05) in December (P < 0.05), the end of the growing season. Fish with low fitness had significantly higher number of parasite than the fish with higher fitness (P < 0.005). Although D. aequans negatively influenced host's condition factor, the impact was not enough to lead mortalities in the cultured sea bass in the Black Sea.