In　this　paper,　we　consider　a　periodic　competitive　stage-structured　Lotka-Volterra　model　with　the　effects　of　toxic　substances.　It　is　shown　that　toxic　substances　play　an　important　role　in　the　extinction　of　species.　We　obtain　a　set　of　sufficient　conditions　which　guarantee　that　one　of　the　components　is　driven　to　extinction　while　the　other　is　globally　attractive.　The　numerical　simulation　of　an　example　verifies　our　main　results.　(C)　2009　Elsevier　B.V.　All　rights　reserved.