The invention relates to a composition for treating keratin substances, in particular the hair, containing, in a cosmetically and/or dermatologically acceptable aqueous medium, at least: a) one zwitterionic polymer resulting from the copolymerization of at least one monomer of formula (I): ##STR1## in which R.sub.1 and R.sub.2, which may be identical or different, represent a hydrogen atom or a methyl radical, and R.sub.3, R.sub.4 and R.sub.5, which may be identical or different, represent a linear or branched alkyl radical containing from 1 to 4 carbon atoms, Z represents an NH group or an oxygen atom, n is an integer from 2 to 5, A.sup.- is an anion derived from an organic or inorganic acid, and at least one monomer of formula (II): R.sub.6 --CH.dbd.CR.sub.7 --COOH (II) in which R.sub.6 and R.sub.7, which may be identical or different, represent a hydrogen atom or a methyl radical; the said polymer being soluble in water to a concentration of 1% by weight at a pH below 6 and at 20.degree. C.; b) one non-amino, non-volatile, water-insoluble silicone with a viscosity of greater than 3.times.10.sup.-5 m.sup.2 /s (300 centistokes).