Polyampholyte gel particles with hydrodynamic radii (Rh) 132 to 87 nm at an isoelectric point were synthesized by aqueous redox polymerization in the presence of sodium dodecylbenzene sulfonate as a surfactant. Acrylic acid and 1-vinylimidazole were respectively used as anionic and cationic monomers, both of which were incorporated into the network of N-isopropylacrylamide cross-linked with N,N'methylenebisacrylamide (BIS). Potentiometric titration gave the following contents (in mol %) of anions (A) and cations (C) in the nanogels: A = 3 and C = 13 for nanogel G(1/4); and A = 14 and C = 12 for nanogel G(1/1). Ampholytic terpolymers with Rh = 23-20 nm, the monomer compositions of which are very close to the corresponding nanogel, were also synthesized without use of cross-linker (BIS). Electrophoretic mobility was measured by electrophoretic light scattering at 25 degreesC as a function of pH and KCl concentration. Dynamic light scattering was performed to determine Rh at the same condition as used in electrophoresis. It was found that the mobilities of the nanogels agree with those of the terpolymers, although there is a big difference in their sizes as well as in their molar masses. A detailed comparison of experimental mobilities with theoretical calculations was made in terms of three different models: free draining model, charged surface model, and Henry's model. It became apparent that the free draining model explains the electrophoretic behavior not only of amphoteric terpolymers but also of polyampholyte nanogels.