Abstract 274 bitterlings (Rhodeus amarus) were parasitologically investigated in Sapanca Lake in October and November 2004. It was seen that the 70 of 169 fish in October and 27 of 105 fish in November were infected by Ligula intestinalis plerocercoids. The average weight of the fish infected by Ligula intestinalis plerocercoids was 2.54 g while this value was 2.98 g in uninfected fish. These results show that one Ligula intestinalis plerocercoid decreases the fish gigantism in the ratio of 14.77 percent.