Terpolymers of acrylic acid (AA), acrylamide (AM), and the zwitterionic monomer 3-[(2-acrylamido-2-methylpropyl)dimethylammonio]-1-propanesulfonate (AMPDAPS) have been prepared by the free-radical polymerization in a 0.5 M NaCl aqueous solution using potassium persulfate as the initiator. The feed ratio of AMPDAPS:AA:AM was varied from 5:5:90 to 40:20:20 mol %, with the total monmer concentration held constant at 0.45 M. Terpolymer compositions were obtained by C-13 NMR. Low-angle laser light scattering provided molecular weights and second virial coefficients which varied from (3.0 to 7.9) X 10(6) and (2.23 to 2.95) X 10(-4) mL mol g-2, respectively. The solubilities of the resulting terpolymers are dependent on pH as well as the amount of AMPDAPS and AA present in the feed. At pH 4 and for higher incorporation of AA and AMPDAPS in the feed (>25 mol %), the terpolymers are insoluble in deionized water and 0.25 M NaCl. At pH 8, all terpolymers are soluble in deionized water and salt solutions. The dilute and semidilute solution behavior of the terpolymers were studied as a function of composition and added electrolytes. Polyelectrolyte behavior was observed for all terpolymers at pH 8 as evidenced by a viscosity decrease in the presence of added electrolytes. The terpolymers exhibit higher viscosities in the presence of NaSCN versus NaCl. Comparison of the solution behavior of the terpolymers to copolymers of AM and AA as well as copolymers of AMPDAPS and AM has been made.