Abstract
1. Large numbers of minnows (Phoxinus phoxinus (L.)) from Fron Goch Pool, near Aberystwyth, have been found to be heavily infested with Diplostomulum pelmatoides Dubois ( = D. phoxini (Faust)), the parasites occurring in very large numbers in the brain, spinal cord and eyes.2. The excretory system of the worm has been worked out in detail. There are fifty-two pairs of flame cells.3. Infested heads, brains and eyes have been fed to the domestic duck and the adult stage of the worm Diplostomum pelmatoides Dubois obtained.4. Some morphological details of the adult worm are described.5. The natural bird host of the parasite in this country is not yet known, nor the stages in the molluscan host.

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