A series of raspberry-shaped composite microgels were synthesized by the seeded emulsion polymerization of styrene with hydrogel particles. Thermoresponsive microgels of poly(N-isopropylacrylamide) cross-linked with N,N'-methylenebis(acrylamide) acted as cores for the polymerization. During the surfactant-free polymerization, the core microgels shrank at 70 degrees C to provide thermoresponsive composite microgels, and the polystyrene particles attached to core microgels became bigger with increasing styrene concentration. Conversely, composite microgels synthesized with sodium dodecyl sulfate (SDS) ([SDS] > 6.5 mM) did not exhibit thermoresponsive deswelling behavior because polystyrene particles covered the core microgels. In particular, polystyrene particles formed composites on the microgel surface as well as inside the microgels when the SDS concentration exceeded a critical value for core microgel swelling at 70 degrees C. A mechanism is proposed based on these results for the seeded emulsion polymerization of water-immiscible monomers with microgels.