The present invention relates to a novel, one-stage method for preparing and isolating 3,4-dichloro-N-(2-cyanophenyl)-5-isothiazolecarboxamide (isotianil), which has microbicidal and plant-fortifying (host defence inducer) properties. In the method according to the invention, the amount of waste from the production process (e.g. solvent) is significantly reduced. The product is provided in high yield and purity (minimal amount of by-products and impurities). Compared to existing preparation methods, the method according to the invention has the advantage of low corrosiveness and high yield and high selectivity.