The combined drug Mebendazole plus Trichlorfon (Telmin plus Trichlorfon, Janssen Pharmaceutica, Beerse/Belgium) has been tested in a field trial against naturally acquired Gasterophilus spp. infestations in horses. 44 foals (1 to 1.5 years old, 350-450 kg body-weight) originating from different endemic areas of Switzerland, have been randomly allocated to two groups as follows: 28 foals were treated with Mebendazole plus Trichlorfon, 16 animals served as untreated controls. The drug (paste) was administered and dosed according to the user's instruction. Macroscopic examinations of the digestive tractus and the larval counting has been performed immediately after slaughtering, i.e. 30 and 33 days after treatment, resp.. The efficacy of the product against Gasterophilus spp. amounts to 97.04% which is considered as highly effective. A further proof for the high efficacy is the observation of a lot of larvae-carrying foals in the control group. Since the larvae of Gasterophilus spp.--according to their life cycle--are essentially found in their hosts between November and February, an appropriate treatment should be performed during that period of time.