The invention relates to isoxazole derivatives of formula (I): ##STR1## wherein: Ar represents phenyl optionally substituted by from one to three groups and wherein two substituents on adjacent positions of the phenyl ring, together with the two atoms to which they are attached, form a 5- to 7-membered ring which is optionally substituted;R represents the hydrogen atom or a group --CO.sub.2 R.sup.3 ;R.sup.1 represents alkyl, haloalkyl or optionally substituted cycloalkyl;R.sup.3 represents alkyl, alkenyl or alkynyl containing up to six carbon atoms which is optionally substituted by one or more halogen atoms;and their use as herbicides.