Abstract: The invention relates to novel 3-methyltetrahydrophthalimides of the formula I in which the substituents R.sub.1 is hydrogen or fluorine, R.sub.2 is halogen, X is O or S and R.sub.3 is C.sub.3 -C.sub.10 -alkenyl, C.sub.5 -C.sub.6 -cycloakly, C.sub.5 -C.sub.6 -cycloalkenyl, C.sub.3 -C.sub.10 -alkynyl, or C.sub.1 -C.sub.10 -alkyl which if desired, is mono- or poly-substituted by halogen, C.sub.1 -C.sub.4 -alkoxy, C.sub.1 -C.sub.4 -alkoxycarbonyl, cyano, C.sub.1 -C.sub.4 -alkylthio or by C.sub.1 -C.sub.4 -alkylthiocarbonyl, to processes for their preparation, and to novel selective herbicidal compositions containing compounds of the formula I.