Ionic polarizability of the kappa-, iota-, and gamma-carrageenans have been investigated by the electric birefringence method. These carrageenan samples were selected because they have different numbers of sulfuric acid residues on their linear polysaccharide chains; that is, their charge densities are different from each other. The analysis of both the polymer concentration dependence and the added salt concentration dependence showed that these three carrageenans have almost the same ionic polarizability irrespective of their different charge densities when they were compared in the same conditions. The results were analyzed by using two groups of theories. That is, one is based on the "loosely bound" counterion polarization, and the other is based on the "condensed" counterion polarization. The ionic strength dependence of the experimentally obtained polarizability could be fitted by Manning’s and Rau and Charney’s "loosely bound" counterion polarization theories.