In　this　study,　an　electrochemical　impedance　biosensor　for　cyanobacterial　toxin　microcystin-LR　(MC-LR)　detection　has　been　developed.　MC-LR　aptamers　were　immobilized　on　the　gold　electrode　through　Au-S　interaction,　in　the　presence　of　target　(MC-LR);　the　binding　of　MC-LR　and　aptamers　probe　led　to　a　complex　formation　change　on　the　electrode　surface　and　resulted　in　the　impedance　decreasing.　The　decrease　rate　had　a　linear　relationship　with　logarithm　of　the　MC-LR　concentration　in　the　range　of　1.0　x　10(-7)-5.0　x　10(-11)　mol/L,　with　a　detection　limit　of　1.8　x　10(-11)　mol/L.　The　sensor　has　good　selectivity　and　stability,　it　has　been　applied　to　detect　MC-LR　in　three　kinds　of　real　water　samples　with　satisfying　results.　(C)　2012　Elsevier　B.V.　All　rights　reserved.