BACKGROUND: Mass mortality of Caspian seal has  occurred in  recent years and canine distemper virus (CDV) has been  identified as the main pathogenic agent in these events. Despite the repetition of this event and the presence of a  large number of dead seals in the coastal region of the Caspian Sea, very little is known about the epidemiological role of these animals in canine distemper virus survival in this area. OBJECTIVES: In this study the frequency of antibody against CDV in unvaccinated rural dogs in the southeast coastal region of the Caspian Sea (Iran) was evaluated by means of serum neutralization test. METHODS: Serum samples (185) were randomly collected from rural dogs from 2008 to 2010. RESULTS: Totally the frequency of positive antibody reaction in animals against CDV was found to be  55.6% (103/185) in 1/32 dilution (CI%95: 47-61). In this study no significant difference in susceptibility was observed between males and females and among different age groups. Moreover, most of the positive cases were observed during the winter. CONCLUSIONS: These results indicate that this virus is present in the ecosystem. Furthermore, there is evidence of previous natural exposure to CDV. This high frequency of antibody in serum samples might be because of previous contact with CDV contaminated corpse of Caspian seal. Dogs' population in rural areas is dense enough to maintain CDV in environment and rural dogs can be a reservoir of infection for urban dogs and wild carnivores.