Associative properties of random copolymers of sodium 2-(acrylamido)-2-methylpropane-sulfonate (AMPS) with N-hexylmethacrylamide (C6MAm) and with N-octadecylmethacrylamide (C18MAm) in aqueous solutions were investigated comparing with those of AMPS copolymers with N-dodecyl-methacrylamide (C12MAm) reported previously. For characterization by fluorescence, all the polymers were labeled with 1 mol % of naphthalene by terpolymerization using N-(1-naphthylmethyl)methacrylamide. The polymers possessing C-6 or C-12 alkyl chains exhibit a strong tendency for intramolecular hydrophobic association to form unimolecular micelles when the CnMAm (n = 6, 12) contents in the polymers are in the ranges of 30-70 and 10-50 mol % for the C-6 and C-12 chains, respectively. A similar tendency was observed for the polymers possessing C-18 chains when the C18MAm contents are in the range of 5-20 mol %, but polymer micelles formed were not completely unimolecular. Both the lower and upper limits of the CnMAm contents for all the three polymers decrease significantly with increasing the length of the alkyl chain. When the CnMAm contents are either lower or higher than these limits, the polymer-bound alkyl chains undergo interpolymer associations. Fluorescence depolarization and H-1 NMR relaxation times indicate that local motions of naphthalene labels and alkyl chains are more pronouncedly restricted in hydrophobic domains formed by longer alkyl chains. An inclination to the self-association of these polymers was discussed in terms of the ratio of the total number of carbon atoms in hydrophobes to the number of SO3- in a polymer chain. A conclusion is that with C-6 chains one needs to incorporate more carbon atoms into a polymer than with C-12 and C-18 chains to attain the same extent of hydrophobe associations. In other words, the CH2 and CH3 residues in the C-6 chain are much less effective than those in the C-12 and C-18 chains for the self-association when compared at the same hydrophobe/charge ratio in a polymer chain, whereas there is no significant difference between those in the C-12 and C-18 chains. Thus, the ability of the C-6, C-12, and C-18 chains per carbon atom to associate is in the order C-18 greater than or equal to C-12 much greater than C-6.