Poly(methyl methacrylate) (PMMA)-polystyrene (PS) composite polymer particles were synthesized in the presence of a surfactant by two-stage seeded emulsion polymerization. The first stage was the synthesis of PMMA particles by soapless emulsion polymerization; the second stage was the synthesis of the PMMA-PS composite polymer particles with the PMMA particles as seeds. In the second stage of the reaction, three kinds of surfactants-sodium laurate sulfate (SLS), polyoxyethylene (POE) sorbitan monolaurate (Tween 20), and sorbitan monolaurate (Span 20)-were used to synthesize the PMMA-PS composite particles. Both the properties and concentrations of the surfactants influenced the morphology of the composite particles significantly. Core-shell composite particles, with PS as the shell and PMMA as the core, were synthesized in the presence of a low concentration of the hydrophilic surfactant SLS. This result was the same as that in the absence of the surfactant. However, a low concentration of Tween 20 led to composite particles with a core/strawberry-like shell morphology; the core region was a PS phase, and the strawberry-like shell was a PS phase dispersed in a PMMA phase. With an increase in the concentration of SLS, the morphology of the composite particles changed from core (PMMA)-shell (PS) to core (PS)-shell (PMMA). Moreover, the effects of a high concentration of Tween 20 or Span 20 on the morphology of the PMMA-PS composite particles were investigated in this study. © 2005 Wiley Periodicals, Inc.