BACKGROUND: The plerocercoid stage of Pseudophyllideancestoda infected a wide range of fresh water fish,particularly the members of the Cyprinidae family. The parasitespecies are the most common pathogens that have severe effectson fish. OBJECTIVES: The aim of the present study is todetermine the occurrence and distribution of the plerocercoid ofDiphyllobothriidae in two freshwater fish from north andnorthwest of Iran. Finally, we discuss the role and dynamics ofthese species of fish in the transmission of infection. METHODS:This study was carried out from September 2011 to September2012on a total of 883 A. bipunctatus and 418 A. brama from northand northwest of Iran. The samples were analyzed to find theplerocercoid infection. RESULTS: From a total number of 883 A.bipunctatus and 418 A. brama fish samples, 558 fish (63.19%)of the former and 67 fish (16.02%) of the latter were infected. Therate of infection was significantly lower in winter (p<0.01). Also,the weight of infected fish was significantly lower than noninfectedones (p<0.01). Moreover, the infection in northwest ofIran was significantly higher than north of Iran (p<0.01).CONCLUSIONS: The family of Diphyllobothriidae is an importantcestode and the prevention programs to break the cycleof infection are essential. More suitable solutions to tackle theproblem, further epidemiological studies on other fresh watersources of Iran are needed.