width could be measured but not be weighed until the age-group of25–29 days old. The infection with O. viverrini was found in all age-groups except in the 9 days old. Mean metacercarial recovery inage-groups 1–29 days old was 0–2.5% which increased to 15.6% inage-group 30–34 days old. The percentage of infected fish in differ-ent age-groups varied from 0 to 100%, from no infection in the groupof 9–10 days old (even after repeating the experiments twice), and100% in age-groups of 30–34, 45–49, 50–54, 55–59 and 60–64. Thepercentage of metacercarial recovery in different age-groups variedfrom 0 to 56%. The lowest intensity of the infection of 0.05 metac-ercariae was in age-group of 5 days, and the highest intensity of 9.5metacercariae was in age-group of 45–49 days (Table 1).