3-Aroylaziridines react with a variety of aryl isothiocyanates in refuxing benzene to give either (a) 4-aroyl-5-arylamino-4-thiazolines by a [2 + 3] cycloaddition and/or (b) 2-arylamino-4-aroyl-4-thiazolines. When the N-substituent of the aziridine is cyclohexyl or isopropyl the product type (a) is isolated in greater quantity, but when the N-substituent is methyl, product type (b) is isolated in greater quantity. The reactions provide convenient one-step syntheses of new 4-aroyl-4-thiazolines.