The invention relates to a process for preparing highly concentrated flowable and pumpable aqueous solutions of betaines by quaternization of compounds containing tertiary amine nitrogen with .omega.-halocarboxylic acids by known processes, wherein 0.1 to less than 3% by weight, preferably 0.1 to 1% by weight, based on the end product, of one or more micellar thickeners are added to the reaction mixture before or during the quaternization reaction.