The interaction between the cetyl trimethylammonium bromide cationic surfactant and the negatively charged poly-(N-isopropylacrylamide-co-acrylic acid) copolymer nanogels was investigated by dynamic light scattering, electrophoretic mobility and potenciometric surfactant activity measurement. The interaction can be divided into three characteristic surfactant concentration ranges. At low surfactant concentration range the nanogel dipersion is stable, the size of the latex particles markedly decreases and the electrophoretic mobility tends to zero with the surfactant concentration. The surfactant binds to the nanogel in form of monomers. Above a certain concentration the nanogel dispersion coagulates. With a further increase of the surfactant concentration a charge reversal of the particles occurs and the nanogel dispersion becomes stable again. The size of the particles increases and the surfactant binding accelerates with increasing equilibrium surfactant concentration, i.e. the binding isotherm shows a new step reflecting collective surfactant binding. In this concentration range the features of the interaction is significantly different from the physical picture previously observed for the mixtures of poly(ethylenimine) and sodium dodecyl sulfate in which case the excess of the surfactant results in a stable colloid dispersion but the particles do not re-swell.