Poly(2-(acrylamido)-2-methylpropanesulfonic acid) sodium salt end-capped with a cholesterol moiety (Chol-PAMPS) was prepared by free radical polymerization of 2-( acrylamido)-2-methylpropanesulfonic acid (AMPS) initiated by a cholesterol substituted azo compound, 4,4＇-azobis(4-cyano-1-cholesteryl)pentanoate, and the associative behavior of Chol-PAMPS in aqueous solution was studied by H-1 NMR, size exclusion chromatography (SEC), static light scattering (SLS), quasielastic light scattering (QELS), and fluorescence probe techniques. For a reference polymer (AIBN-PAMPS), polyAMPS of a similar molecular weight with that of Chol-PAMPS was prepared in the presence of 2,2＇-azobis(2-methylpropionitrile) as an initiator. The degree of polymerization (DP) of Chol-PAMPS was estimated to be about 70 from a H-1 NMR spectrum (in DMSO-d(6)) by assuming disproportionation for the termination. This DP value agreed fairly well with that estimated by SEC in water/acetonitrile using sodium poly(styrenesulfonate)s as standards. QELS indicated that Chol-PAMPS formed multipolymer aggregates at polymer concentrations (C-P) higher than 0.5 g/L in 0.1 M NaCl. Fluorescence emission and excitation spectra for pyrene probes solubilized in the aggregates of Chol-PAMPS suggested the presence of a critical micelle concentration (cmc) around C-P approximate to 0.6 g/L in water. Above this C-P, the micellelike aggregates coexist with unimers over a wide range of C-P. in the C-P regime of 1.0-5.0 gn, hydrodynamic radii for the aggregates were practically constant at about 50 nm. However, when C-P was increased beyond this C-P regime, the size increased considerably with C-P. Given DP approximate to 70 for Chol-PAMPS, these sizes for the aggregates are obviously too large for a single spherical micelle with cholesterol groups in the core and extended polyAMPS chains in the corona. The structures of these multipolymer aggregates remain to be an open question, but these results are explained by considering that some polymer chains possess cholesterol groups at both chain ends and spherical micelles are bridged by these polymer chains.