In　this　paper,　we　consider　a　delay　differential　equations　model　of　plankton　allelopathy.　Using　an　iterative　method,　the　global　stability　of　the　interior　equilibrium　point　of　the　system　is　investigated,　which　shows　that　toxic　substances　are　harmless　for　the　stability　of　the　interior　equilibrium　point.　The　results　we　obtained　improve　and　complement　those　of　Chattopadhyay　[J.　Chattopadhyay,　Effect　of　toxic　substances　on　a　two-species　competitive　system,　Ecol.　Model.　84　(1996)　287-289],　Samanta　[G.　P.　Samanta,　A　two-species　competitive　system　under　the　influence　of　toxic　substances,　Appl.　Math.　Comput.　216　(2010)　291299]　and　Mukhopadhyay　et　al.　[A.　Mukhopadhyay,　J.　Chattopadhyay,　P.　K.　Tapaswi,　A　delay　differential　equations　model　of　plankton　allelopathy,　Math.　Biosci.　149　(1998)　167-189].　We　also　present　some　examples　to　verify　our　main　result.　(C)　2012　Elsevier　Inc.　All　rights　reserved.