A striking difference between the reactions of 3-methylbenzothiazoline derivatives with trimethyloxonium tetrafluoroborate and with methyl iodide has been found. Especially, by the treatment of 2, 2, 3-trimethylbenzothiazoline with methyl iodide, N-methyl-N-(o-methylthiophenyl)-N-isopropylideneiminium iodide has been obtained. Mechanism for the reaction was also proposed.