Anionic polymerizations of 1,1-dimethylsilacyclobutane, 1,1-diethylsilacyclobutane and 1-methyl-1-phenylsilacyclobutane were investigated. Addition of 5 mol % of butyllithium to a solution of 1,1-dimethylsilacyclobutane in THF-hexane (1 : 1) at -48 degrees C provided poly(1,1-dimethylsilabutane) in 99% yield. M-n and M-w/M-n of the obtained polymer were 2400 and 1.10. This polymerization proceeded with a living nature. M-n increased in proportion as the yield of polymer increased. Addition of the second fresh feed of the monomer to the reaction mixture restarted polymerization of the second monomer at the same rate as in the initial stage. Addition of styrene to the living poly(1,1-dimethylsilabutane) provided a poly(1,1-dimethylsilabutane-b-styrene) block copolymer. It was also found that a polymerization of 1,1-diethylsilacyclobutane in THF-hexane at -48 degrees C showed a living nature. In contrast, a polymerization of 1-methyl-1-phenylsilacyclobutane in THF at -78 degrees C did not show a living nature.