Attempts were made to induce acquired immunity against Opisthorchis viverrini infection in hamsters by immunizing them with aqueous somatic extract and metabolic products of adult worms, crude adult worm homogenates and metacercarial somatic extracts via either the intraperitoneal or combined intraperitoneal and oral routes. These procedures failed to stimulate significant protective response in animals that had never been exposed to O. viverrini. However, the protective response reached a significant level (30% worm reduction) in hamsters that had been infected with a small member of flukes prior to immunization with aqueous somatic extract of adult worms. Although these findings indicate that it may be possible to reduce reinfection in people in the endemic area by immunization, it appears that a better method currently available for the control of O. viverrini infection is health education aimed at changing food habits and improving sanitation and personal hygiene.