Polyisoprenes were found to form inclusion complexes with cyclodextrins with high selectivity to give crystalline compounds. beta-Cyclodextrin formed complexes only with polyisoprene of low molecular weight, whereas T-cyclodextrin formed complexes with polyisoprenes of high molecular weight. alpha-Cyclodextrin did not form complexes with polyisoprene of any molecular weight. The yields of gamma-cyclodextrin complexes increased with increasing molecular weights of the polyisoprenes and reached a maximum of around several thousands, and then decreased.