A series of 3(2H)-oxo-N-(substituted phenyl)-4,5,6,7-tetrahydro-1,2-benzisothiazoles (2) were obtained via four reaction steps starting from 2-chlorocyclohexene-1-carboxylic acid. The sulfur atom of 3-oxo-4,5,6,7-tetrahydro-1,2-benzisothiazoles (2) was oxidized with an equimolecular amount of 3-chloroperbenzoic acid (3-CPBA) in chloroform to give the corresponding 3-oxo-4,5,6,7-tetrahydro-1,2-benzisothiazole-1-oxides (3). Oxidation of 2 with two moles of 3-CPBA afforded 3-oxo-4,5,6,7-tetrahydro-1,2-benzisothiazole-1,1-dioxides (4). Phytotoxic activities of the compounds (2-4) synthesized were assayed by inhibition of protoporphyrinogen-IX oxidase isolated from corn as well as by growth inhibition, chlorophyll decrease and peroxidative destruction of cell membranes of the green microalga Scenedesmus acutus. Among the compounds (2-4), 4 showed the strongest activities according to all phytotoxic parameters, exhibiting phytotoxicities characteristic of peroxidizing herbicides. 3(2H)-oxo-2-[4-chloro-3-(isopropoxycarbonyl)phenyl]-4,5,6,7-tetrahydro-1,2-benzisothiazole-1,1-dioxide (4h) was the strongest of the compounds 4 tested.