BACKGROUND: Dirofilaria immitis is a nematode that is highly prevalent in dogs and it can cause dirofilariosis. The disease is transmissible to human, so it is important in terms of public health. OBJECTIVES: The aim of the present study was to diagnose Dirofilaria immitis infection in the dog population of urban and rural areas of Ahvaz by counterim-munoelectrophoresis and compared with knott test. METHODS: In the present study, serum of 200 dogs (80 urban and 120 rural), were evaluated for detection of Dirofilaria immitis infection, in Ahvaz area, Southwestern Iran. Counterimmunoelectrophoresis and modified knott's test were conducted on all blood samples to trace the antibody and microfilariae. Results: Using counterimmunoelectrophoresis test, 9.5 percent of dogs (19 cases) were infected, but in modified Knott test, positive cases were detected 8 percent (16 cases). Counterimmunoelectrophoresis test showed three more positive cases (one urban and two rural dogs) compared with the modified Knott test, which probably was due to occult infection. However, the difference was not significant (p>0.05). Based on the results of counterimmunoelectrophoresis test, 6.25 percent of male and zero percent of female dogs in urban areas and 13 percent of male and 7.6 percent of female dogs in rural areas were infected respectively. The highest prevalence of infection in 6 year-or-more age groups was 23.3 and 14.29 percent in urban and rural dogs respectively. Statistical analysis did not show any significant relationship between infection and factors such as age and sex of the studied dogs (p>0.05). CONCLUSIONS: It is proposed that for more accurate diagnosis of dirofilariosis, counterimmunoelectrophoresis test and the modified knott's test be used simultaneously.