Flounder Platichthys flesus (L.) (1509 specimens) was sampled randomly between April 1998 and March 2000 in the Gulf of Gdańsk. The sex, age, and parasites were determined. The prevalence, intensity, mean intensity, and relative density of the infection of the detected parasites were calculated according to host sex and age. Susceptibility to infection was related to host age. Each parasite species had its own host sex preferences. The results indicated that host age and sex could play a major role in the determination of parasitic infection in flounder.