Ligula intestinalis lives in the abdominal cavity of fish, causing detrimental effects in the host. Parasitizing by L. intestinalis is known as ligolosis which is one of the most common diseases in freshwater fishes around the world, especially in lakes and reservoirs. In this study, prevalence of parasite in Petroleuciscus esfahani , an endemic species in the Zayandeh- Roud River, and its effects on some growth and reproductive parameters were investigated. Sampling was carried out in two different stations. The first station (polluted) was located near Zobahan factory waste refinery, whereas the second station (control) was near Safaieh Bridge, between Zarin shahr and Mobarakeh Cities. Sampling was done at 3 different times, the first one before the spawning season, March 2012, and the other two during April and May 2012, respectively. A total of 290 individual fish were studied. From the 175 fish caught at the polluted station, 59 individual contained the plerocercoid of the parasite, with an occurrence of 33.71% or one third of the total fish sampled. Parasited and non-parasited fish from the polluted station were evaluated and compared with each other and then with fish from the control station, in order to detect possible effects of other ecological factors on the studied parameters. The average weight was significantly different between fish from polluted and non-polluted stations, which could be explained by other factors, such as environmental contaminants. The results showed that GSI, HSI and liver glycogen were decreased due to the presence of Ligula. The significant fecundity decrease in fish from the polluted station, in comparison to fish from control station, can be explained by the probable interaction between the presence of the parasite and other factors, such as the environmental pollutants. Some parasited fish showed hemarrage and leucocytes accumulation in liver, as well as oocyte degeneration, amorphism and atrophy in the gonad tissue of the females, due to damage by physical pressure caused by the presence of the parasite. The condition factor was not affected by Ligula infection. Our results suggest that the parasite can have negative effects on the reproductive parameters in P. esfahani , threatening the population of this species in Zayandeh Roud River. I addition to the environmental pollutants, stagnant water and presence of aquatic birds in the sampling station can increase the susceptibility of fish to the parasite. Therefore, diminishing or stopping the entrance of industrial pollutants to the river, as well as, controlling the intermediate hosts, could decrease the prevalence of this parasite. Key words : Petroleuciscus esfahani , Ligula intestinalis , growth, reproduction, liver glycogen, Zayandeh-Roud River.
