A magnetic poly(methyl methacrylate) (PMMA) composite latex was prepared by soapless emulsion polymerization in the presence of ferrofluid, and the ferrofluid was prepared by means of a coprecipitation method. The effects of various polymerization parameters, such as the monomer concentration, ferrofluid content, and initiator concentration, on the conversion curve and particle size of the magnetic composite latex particles were examined in detail. The results showed that two nucleation mechanisms were involved according to the polymerization conditions. In the monomer-rich and less ferrofluid system, self-nucleation of PMMA was dominant over the entire course of emulsion polymerization. In the ferrofluid-rich system, seeded emulsion polymerization was the main course to form the magnetic composite latex particles. (C) 2004 Wiley Periodicals, Inc.