Half of the young Pekin ducks used in four experiments were given 250 cysts of Sphaeridiotrema globulus each. Sixteen days later all of the birds were given 1000 cysts each. When the birds were killed or died after 4 days, 11 of the 18 birds with initial infections were free of trematodes while all 17 of those without initial infections were parasitized. The control ducks harbored 6911 flukes, 17 times the number found in the previously infected birds.