A uniform spherical polychloromethylstyrene (PCMS) microgel whose average diameter was 2.3 mu m was prepared by dispersion copolymerization of chloromethylstyrene and divinylbenzene in ethanol-DMSO (25/3 v/v) in the presence of polyvinyl-pyrrolidinone. When the PCMS microgel was treated with an excess amount of methyl thioglycolate (MTG) in the presence of 1,8-diazabicylco[ 5.4.0]undec-7-ene (DBU) at room temperature in THF for 4 h, sulfenylated microgel was obtained. The introduction ratio of MTG corresponded well to the amount of DBU used. A transmission electron microscope (TEM) photograph and thermal analysis of the PCMS microgel partially modified by MTG showed that it had a core-shell structure, that is, an MTG-modified shell and an unchanged core. Since the reaction of the chloromethyl group and MTG was a diffusion-limited one, MTG was introduced into the PCMS microgel from the outer side layer by layer. The PCMS microgel in which 52% of the MTG was introduced was treated with an excess amount of pyridine in DMAc at 50 degrees C for 48 h followed by acid-catalyzed hydrolysis in dioxane-water at 80 degrees C for 48 h to give a zwitterionic microgel that formed a stable suspension.