In a process for preparing 2-substituted 1-(acylaminoalkyl)-2-imidazolines of the general formula I ##STR1## where R is an alkylene radical and R.sup.1 is a fatty acid radical, for example a fatty acid and a dialkylenetriamine are reacted with one another in a molar ratio of (1.8 to 2.0):1 by initially introducing one starting material at a temperature T.sub.1 and metering in the second starting material as a liquid, maintaining in the reaction mixture during the addition a temperature T.sub.1 and after the addition until the start of the cyclization a temperature T.sub.2 and distilling off the resulting water or alcohol via a rectification column and, after the acylation product has been formed, cyclizing in a conventional manner, T.sub.1 being a temperature of 170.degree. C. to the boiling point of the initially introduced starting material, but not higher than 250.degree. C., and T.sub.2 being a temperature of 170.degree. to 250.degree. C.