A total of 540 individuals of Capoeta capoeta gracilis were collected from two sampling sites, one each from upstream (36°11Ã?Å Ã?¹24.91"N, 53°19Ã?Å Ã?¹32.13"E) and downstream (36°16Ã?Å Ã?¹15.36"N, 53°12Ã?Å Ã?¹51.44"E) of Shahid-Rajaei dam on Tajan River in February 2010. In the present study, a total of 545 individuals of Tracheliastes polycolpus were found in C.c. gracilis. The prevalence (P), mean intensity of infection (MI), range and mean abundance (MA) of the parasite are as follow: P= 46.7%, MI= 2.2±1.9, MA= 1.0±1.7. The parasite was found in highest prevalence (23.7%) on dorsal fin than on the other fins. The infestation of pelvic and pectoral fins were the second and third highest respectively. The prevalence, mean intensity of infection and mean abundance of T. polycolpus were significantly higher in spring and summer than in autumn and winter. There were also found significant differences in infestation with T. polycolpus between different age and sex classes of fish and different localities.
