Anilides bearing a guanidino, thioallophanato, isothioallophanato or amidino group in the ortho position were synthesised (6) for investigations of their anthelmintic effects, and tested against sheep trichostrongylides. The most active anilides are those containing a 5-alkylthio, 5-alkylsulphinyl (alkyl = C3H7, C4H9), 5-phenylthio, 5-phenylsulphinyl or 4-phenylsulphonyloxy group, and a guanidino group substituted at both nitrogen atoms by a methoxycarbonyl group. The most active anthelmintic anilides are the form-, propion-, butyr- and methoxy-acetanilides (e.g. 60, 61, 66, 95, 100, 110). Of the guanidines unsubstituted at N' and N", the 2-guanidino-5-phenylthio-2-methoxyacetanilide (123) and the 2-guanidino-5-phenylsulphinyl-2-methoxyacetanilide (152) were more active than parbendazole. From the class of bismethoxycarbonylguanidines (6 alpha), which are also the most interesting products in terms of stability, febantel (110, Rintal) has been introduced into veterinary practice for the treatment of nematode infections in horses, cattle, sheep and swine.