Parasites influence population structure and population control of fishes by effecting their survival, reproduction and migration behavior. This study has been carried out to focus on relationships of the host and pathogen, emphasizing on the Posthodiplostomum cuticola in native fishes of Zaringol stream over a period of a year (December, 2009 to October, 2010). 1346 specimens were caught by electrofishing (1.7 KW, 10 A and 200-300 V) from 14 research stations along the stream seasonally. Fish health condition was checked through observing the surface of their body and fish populations were estimated by Lecren method. Sampling from Macro-invertebrates also was done by surber sampler (30cm×30cm) and were fixed in formalin (4%). Results showed that the maximum frequency of fishes (individual per square meter) belongs to Capoeta capoeta gracilis (1.15) at station 12 (1.309) in summer (2.36). The results obtained for infested fish (relative frequency) in Neogobius fluviatilis Pallasi at station 12 in winter were 90.9,46 and 53, respectively. Maximum frequency of 8 snails families (individual per square meter) was observed at station 12 (2.18) in winter (1.4). Results show that distribution of fish species based on their biological demand, vegetation cover around the stream, low current velocity at station 12 side by side desirable water temperature, and occurrence of flood in winter have important effect on first intermediate host (snail) assemblage and help the parasite complete its life cycle.