A copolymer produced by the copolymerization of a monomer mixture comprising at least 30 weight percent of at least one organic ethylenically unsaturated carboxylic acid, at least 30 weight percent of at least one N-alkyl acrylamide the alkyl group of which contains 8 to 30 carbon atoms in the alkyl chain and 0 to 40 weight percent of at least one hydrophilic unsaturated monomer as well as a corresponding copolymer the carboxylic groups of which are completely or partially salified. The preparation and the use of the copolymer as a thickening agent in compositions having a high amount of a surfactant such as a shampoo are also disclosed.