Gyrodactylus elegans on goldfish, (Carassius auratus) from a commercial farm were resistant to recommended dosages of dimethyl (2,2,2-trichloro-1-hydroxyethyl) phosphonate. Controlled experiments suggest that a dosage 100 times the commonly recommended minimal dosage (.25 mg/l) was required to remove trematodes. A hypothesis is proposed to account for the development of drug resistant trematodes based on the life cycle of the parasites and continual drug exposure.
