Naphthostyril is obtained in high space/time yields by reacting 1-naphthyl isocyanate with an anhydrous aluminium halide in an inert solvent, carrying out the reaction in a homogeneous solution using 700-1.800 ml of solvent per mol of 1-naphthyl isocyanate in the temperature range from 140.degree.-160.degree. C. and -- after hydrolysis of the reaction mixture -- separating the phases. Naphthostyril is an intermediate for preparation of dyestuffs.