Field rodents involved in ecological food chains and which are the prey of carnivores in the natural environment may serve as reservoir hosts for Toxoplasma gondii infection in humans, however, no data has been published to date in Korea. A total of 1,008 Apodemus agrarius, a dominant species of field rodents in Korea, were trapped at various locations around the country, and their serum antibody (IgG) levels to T. gondii were examined by ELISA. The mean absorbance was 0.11, and fifteen samples (1.49%) showed positive titers from 0.18 to 0.59. The seropositive samples were analyzed by immunoblot. Five of them showed reactive bands to T. gondii water soluble antigens of 30, 35, and 43 kDa. This immunoblot analysis showed very similar patterns to that obtained using sera of experimentally infected mice with T. gondii. The present study presents indirect evidence of the existence of T. gondii in field rodents in Korea.