A total of 1509 specimens of flounder Platichthys flesus (L.) were sampled in the period from April 1998 to March 2000 from the Gulf of Gdańsk. The seasonal occurrence of all parasite species was calculated. The results indicated that some parasite species occur seasonally (Diplostomum sp., Raphidascaris acus, Dichelyne minutus, Echinorhynchus gadi and Pomphorhynchus laevis), while others do not (Trichodina borealis, Hysterothylacium aduncum, Cucullanus heterochrous). Since some parasites occurred year-round at low levels of infection, it was difficult to determine if they exhibited seasonality or not (Glugea anomala, Gyrodactylus unicopula, Diplostomum mergi, Bothriocephalus scorpii and Lampetra sp.). Gyrodactylus flesi was noted occasionally.