Thermosensitive core-shell nanoparticles were synthesized by semicontinuous heterophase polymerization of styrene, followed by a seeded polymerization for forming a shell of poly(N-isopropyl acrylamide) (PNIPAM). Nanoparticles characterization by scanning transmission electronic microscopy showed core-shell morphology with average particle diameters around 40 nm. An inverse dependence of the particle size with temperature in the range 20-55 A degrees C was identified by quasielastic light scattering measurements. As was expected for core-shell particles with PNIPAM as the shell, a volume phase transition near 32 A degrees C was detected. In spite of thermosensitive properties of core-shell nanoparticles synthesized here, the volume percentage loss values were not so high, probably due to their relatively low content of PNIPAM.