Canine ehrlichiosis is a zoonotic rickettsial disease transmitted by ticks. In the present study, 198 companion dogs of different ages were examined for serum antibody detection against Ehrlichia canis by means of immunochromatography assay. The dogs were selected among referred cases to Veterinary Hospital of Shahid Chamran University of Ahvaz, Southwestern Iran from November 2008 to March 2010. The studied dogs were classified based on age, sex, breed, region and season. Nineteen of 198 serum samples (9.6%) had antibody against E. canis. Prevalence was significantly higher in adult dogs more than 3 year-old (16.18%) (P= 0.002) and 1 – 3 years (11.86%) (P= 0.016) compared with young dogs less than 1 year-old (1.41%). Prevalence was higher in male dogs (10.62%) than female dogs (8.24%), in the summer (11.32%) and west region (11.11%). There were not significant differences between the prevalence of infection and host gender, season and region (P>0.05). Typical morulae of E. canis were observed in monocytes of four infected dogs (2.02%). Five out of 24 (20.83%) of the thrombocytopenic dogs and 14 out of 174 (8.05%) of the non-thrombocytopenic dogs were positive for ehrlichiosis. Of 19 seropositive dogs, six (31.58%) had anemia, four (21.05%) hypoalbuminemia and five (26.32%) leukopenia. There were not statistically significant differences between hematological findings and prevalence of infection (P> 0.05). This is the first report indicating the presence of E. canis in companion dogs of Ahvaz district. However, the sources of infection in these dogs were not clear. Finally, the role of companion dogs in the epizootiology of E. canis infection needs to be further explored.