a hair - conditioning resin composition prepared by modifying a copolymer resin with a zwitterionizing agent is described , said copolymer resin being prepared by copolymerizing in a hydrophilic solvent a polymerizable mixture containing 65 - 90 % by weight of a polymerizable vinyl monomer having the general formula ## str1 ## wherein r 1 represents a hydrogen atom or a methyl group , r 2 represents an alkylene group having from 1 to 4 carbon atoms , r 3 and r 4 each represents an alkyl group having from 1 to 4 carbon atoms , and a represents an oxygen atom or an nh group , 10 - 35 % by weight of polymerizable vinyl monomers having the general formula ## str2 ## wherein r 5 represents a hydrogen atom or a methyl group , and r 6 represents a saturated or unsaturated alkyl group having from 12 to 24 carbon atoms , and 0 - 25 % by weight of other polymerizable vinyl monomers , said zwitterionizing agent having the general formula xr . sub . 7 coob wherein r 7 represents an alkylene group having from 1 to 4 carbon atoms , x represents a bromine , chlorine or iodine atom , and b represents an alkali metal ion , an ammonium salt or an amine salt . the hair - conditioning resin compositions of the present invention have satisfactory cosmetic properties such as voluminousness , combing - out , softening , moderately moistening , and wavesetting .