there is disclosed a novel isothiazolone derivative of the formula : ## str1 ## wherein x 1 and x 2 are the same or different and are independently hydrogen , chlorine or bromine ; and r is haloalkoxy . the isothiazolone derivative is effective as a microbicide . also disclosed are a microbicidal composition containing the isothiazolone derivative as an active ingredient , and an intermediate compound for use in the production of the isothiazolone derivative .