The invention is related to a novel phenylacetylene derivates of formula: ##STR1## wherein X represents hydrogen, halogen, optionally substituted C.sub.1-6 alkyl; Y represents hydrogen, halogen, cyano, nitro, optionally substituted C.sub.1-6 alkyl, C.sub.1-6 alkoxy, C.sub.3-8 cycloalkoxy, optionally substituted C.sub.1-6 alkylthio, C.sub.3-8 cycloalkylthio, C.sub.1-6 alkylsulfinyl, optionally substituted C.sub.1-6 alkylsulfonyl, C.sub.3-8 cycloalkylsulfonyl, C.sub.1-6 alkylsulfonyloxy, optionally substituted sulfonamide, optionally substituted carboxamide, etc., or Y may form together with Z which is bonded to the vicinal carbon atom a 5- or 6-membered ring; n is 0 or 1; Z represents hydrogen or halogen; m is 1 or 2; R represents hydrogen, halogen, C.sub.1-6 alkyl, tri(C.sub.1-6 alkyl)silyl, COOR.sup.c or CONR.sup.d R.sup.e ; Q represents a heterocyclic group, and their use as herbicid.