A　new　＇　signal-on＇　aptasensor　for　ultrasensitive　detection　of　Ochratoxin　A　(OTA)　in　wheat　starch　was　developed　based　on　exonuclease-catalyzed　target　recycling.　To　construct　the　aptasensor,　a　ferrocene　(Fc)　labeled　probe　DNA　(S1)　was　immobilized　on　a　gold　electrode　(GE)　via　Au-S　bonding　for　the　following　hybridization　with　the　complementary　OTA　aptamer,　with　the　labeled　Fc　on　S1　far　from　the　GE　surface.　In　the　presence　of　analyte　OTA,　the　formation　of　aptamer-OTA　complex　would　result　in　not　only　the　dissociation　of　aptamer　from　the　double-strand　DNA　but　also　the　transformation　of　the　probe　DNA　into　a　hairpin　structure.　Subsequently,　the　OTA　could　be　liberated　from　the　aptamer-OTA　complex　for　analyte　recycling　due　to　the　employment　of　exonuclease,　which　is　a　single-stranded　DNA　specific　exonuclease　to　selectively　digest　the　appointed　DNA　(aptamer).　Owing　to　the　labeled　Fc　in　close　proximity　to　the　electrode　surface　caused　by　the　formation　of　the　hairpin　DNA　and　to　the　analyte　recycling,　differential　pulse　voltammetry　(DPV)　signal　could　be　produced　with　enhanced　signal　amplification.　Based　on　this　strategy,　an　ultrasensitive　aptasensor　for　the　detection　of　OTA　could　be　exhibited　with　a　wide　linear　range　of　0.005-10.0ngmL-1　with　a　low　detection　limit　(LOD)　of　1.0pgmL-1　OTA　(at　3σ).　The　fabricated　biosensor　was　then　applied　for　the　measurement　of　OTA　in　real　wheat　starch　sample　and　validated　by　ELISA　method.　©　2011　Elsevier　B.V.