We performed a serological survey of Toxocara canis infection in junior high school students from three districts in northern Sulawesi. Almost all of the 117 subjects from two rural districts near Manado allowed dogs in their houses, and there was an 84.6% prevalence of T. canis infection in this group. Fifty-three subjects (45.3%) had serum samples with a high titer of specific anti-Toxocara antibody. By contrast, 41 students tested in one urban district showed a 12.2% prevalence. To confirm the clinical symptoms of visceral larva migrans (VML) and ocular larva migrans (OLM) caused by Toxocara, we administered a questionnaire survey, serological liver function tests, and an ophthalmoscopic examination in 34 subjects having high anti-Toxocara antibodies. One rural district showed a high prevalence; 58 out of 71 subjects (81.7%) had a high titer of anti-Toxocara antibodies according to a plate-ELISA test, although none showed clinical signs. Five of these subjects exhibited hypereosinophilia. These results indicated that T. canis infection in northern Sulawesi is latent in many more cases than previously estimated, and suggest that people living in environments polluted by Toxocara eggs become easily infected with T. canis and show a high prevalence of infection.