For the subdueing of the clinical form of the synagamosis of pheasants we successfully applied Mebendazole in the dose of 120 mg kg-1 of the feed. Because of the lack of drug we administered the dose of 120 mg kg-1 of the feed in one group in the course of seven days to a number of animals, the others (approx. 4000 animals) received the drug only in the course of three days. On the third to fourth day after the application of the preparation the clinical symptoms disappeared and mortality stopped. To the group intended for dissecting for the evaluation of the effectiveness of Mebendazole, the dose of 120 mg kg-1 of the feed was applied in the course of 13 days. From the fifth day after the beginning of the application we found no gapeworm (Syngamus trachea) in the trachea, and the clinical symptoms disappeared completely from the seventh day onwards. In the control group mortality continued and in the trachea we found mature parasites numbering from 1 to 13 pairs. Thus we consider the dose of 120 mg kg-1 of the feed to be suitable, though it is necessary, by means of further investigation, to determine the most favourable time of administration, both for the prophylactic dehelminthization in dependence on the concrete ecological conditions in the system of rearing, and also for the therapy of the clinical form in dependence on the stage of the pathological process and of the epizootological situation.