A new class of herbicidal compounds consisting of 1-alkyl- and 1,1-dialkyl-3-(4-substituted-3-alkoxy-5-isothiazolyl)ureas and N-(4-substituted-3-alkoxy-5-isothiazolyl)alkanamides, in which the 4-substituent consists of alkoxycarbonyl, cyano and carbamoyl, exhibits preemergence and postemergence herbicidal activity, controlling effectively the growth of a wide spectrum of grassy and broad-leaved plant species. The synthesis of members of this class is described in detail, and the utility of representative compounds is exemplified.