Helminth parasites of sand goby (Gobius fluviatilis Pallas, 1811) from Lake Uluabat were investigated on a monthly or bimonthly basis between April 1998 and November 1999. Four species of helminth parasites were recorded in 98 out of 134 fish: Gyrodactylus gobii Shul'man, 1953; Bothriocephalus acheilognathi Yamaguti, 1934; Ligula pavlovskii Dubinina, 1959; Eustrongylides excisus Jagerskold, 1909. They were evaluated with respect to seasons and host size. While G. gobii appeared to infect the gills and the skin of host fish, B. acheilognathi established itself in the intestines, and E excisus and L. pavlovskii in the body cavity. A total of 789 specimens of G. gobii were detected especially on smaller host fish during the spring period (prevalence 25.3%, mean intensity 23.21 +/- 9.17 specimens/fish). Although 184 B. acheilognathi appeared on host fish of various sizes and in all seasons, the mean intensity of infection was relatively low (prevalence 34.3%, mean intensity 3.98 +/- 3.15 specimens/fish). Concerning L. pavlovskii, the second cestode species, only 9 parasites were found in medium-sized host fish during the summer, autumn and winter (prevalence 3.7%, mean intensity 1.60 +/- 0.89 specimens/fish). A total of 108 parasite specimens of E. excl. sus was found on 40 host fish in the summer and autumn (prevalence 29.8%, mean intensity 2.70 +/- 2.29 specimens/fish). The present study gives the occurrence, density, seasonal changes, and parasite-host size relationship for a new locality -Lake Uluabat- as a contribution to the geographical distribution of G. gobii and E. excisus in sand goby.