A herbicidal alkanoic acid derivative of the formula: ##STR1## wherein R is ##STR2## wherein R.sup.3 is a hydrogen atom, a halogen atom, a halogen-substituted alkyl group, an alkyl group, a cycloalkyl group, an alkylthioalkyl group, a hydroxyalkyl group, a hydroxyl group, a cyano group, an acyloxyalkyl group, a thienyl group, a naphthyl group, a dihydronaphthyl group or ##STR3## wherein R.sup.8 is a hydrogen atom, a halogen atom, a nitro group, an alkyl group, an alkoxy group or --S(O).sub.n R.sup.9 wherein R.sup.9 is an alkyl group, and n is an integer of from 0 to 2, m is an integer of from 0 to 2, each of R.sup.2 and R.sup.4 which may be the same or different is a hydrogen atom or an alkyl group, or R.sup.2 and R.sup.4 form together with the adjacent carbon atom a 3-, 4-, 5- or 6-membered ring which may contain an oxygen atom and may be substituted by one or two alkyl groups, each of R.sup.5 and R.sup.6 which may be the same or different is a hydrogen atom or an alkyl group, R.sup.7 is an alkyl group or a phenyl group, or R.sup.6 and R.sup.7 form --(CH.sub.2).sub.l -- wherein l is an integer of 3 or 4 which may be substituted by one or two alkyl groups, or R is an alkenyl group, a dihydronaphthyl group, a tetrahydronaphthyl group, a 1-oxo-1,2,3,4-tetrahydronaphthyl group, a 1,2-epoxycycloalkyl group or an indanyl group which may be substituted by an alkyl or alkoxy group; R.sup.1 is a hydrogen atom, an alkyl group, an alkenyl group, an alkynyl group, a phenyl group, an alkylideneamino group, an alkoxyalkyl group, an alkoxycarbonylalkyl group, a halogen-substituted alkyl group, a cycloalkyl group, a nitro-substituted phenylthioalkyl group, a halogen atom or a benzyl group which may be substituted by an alkyl or alkoxy group; or R and R.sup.1 form a ring; A is an alkyl group, an alkoxy group, an alkylthio group, a halogen atom, a halogen-substituted alkoxy group, an amino group, an alkylamino group or a dialkylamino group; B is a hydrogen atom, an alkyl group, an alkoxy group or a halogen-substituted alkoxy group; X is an oxygen atom or a sulfur atom; and Z is a methine group or a nitrogen atom; and a salt thereof.