Roach Rutilus rutilus (Linnaeus, 1758) is a very common fish species for Turkish inland waters, especially in Büyükçekmece Reservoir (İstanbul). The species is one of the most caught fish species and holds economic consumption value even though it is mostly infected by Ligula intestinalis (Linnaeus, 1758), a pseudophyllidean cestode causing severe pathological effects on fishes. The aim of the study was to determine the presence of L. intestinalis plerocercoids in its fish host R. rutilus and investigate the effects of the parasite on the condition of the fish. With this purpose, the fish specimens have captured from Büyükçekmece Reservoir with using gillnets having different mesh sizes (10×10 mm, 20×20 mm, 30×30 mm, 40×40 mm and 50×50 mm) from March 2009 to February 2010. The fork length and body weight of fish specimens (n=1857) were varied between 6.0-29.2 cm and 2.53-561.00 g, respectively. In total, 4.52% of specimens were infected by the plerocercoids. Infection by L. intestinalis was observed during summer, autumn and winter months but not spring. Parasite-host index (PSI %), prevalence (%) and mean intensity of plerocercoids for infected fishes were calculated, monthly. PSI (%) was estimated maximum in January as 18.49% while prevalence (%) was 32.31% in July and mean intensity of plerocercoids is 6.0 in October. Statistical-ly significant differences between K values of non-infected and infected specimens among length groups and months were recorded (Mann-Whitney U Test, p<0.05). Results showed that, L. intestinalis plerocer-coids seem to be affected significantly on the condi-tion and body health of its host R. rutilus.