A　label-free　electrochemical　aptasensor　was　developed　for　selective　detection　of　saxitoxin　(STX).　It　is　taking　advantage　of　target-induced　conformational　change　of　an　STX-specific　aptamer　when　it　binds　to　the　toxin.　A　monolayer　of　octadecanethiol　was　deposited　on　a　gold　electrode,　and　then　coated　with　a　film　of　multiwalled　carbon　nanotubes　(MWCNTs)　to　which　the　aptamer　was　covalently　conjugated.　Methylene　blue　(MB)　was　electrostatically　anchored　on　carboxylated　MWCNTs　and　used　as　the　electrochemical　indicator　that　produced　a　strong　differential　pulse　voltammetric　signal　in　the　absence　of　target　(STX).　If,　however,　STX　binds　to　its　aptamer,　this　triggers　a　conformational　change　of　the　aptamer　and　results　in　the　establishment　of　a　barrier　for　heterogeneous　electron　transfer.　The　oxidation　peak　current　of　MB,　acquired　at　-0.27　V　(vs.　Ag/AgCl),　linearly　decreases　with　increasing　concentrations　of　STX　in　the　0.9　and　30　nM　concentration　range.　The　detection　limit　is　0.38　nM.　Marine　toxins　that　maybe　present　along　with　STX　do　not　interfere　even　if　they　have　a　similar　chemical　structure.　The　assay　was　applied　to　the　determination　of　STX　in　mussels　samples　and　was　found　to　be　acceptably　accurate.　Hence,　the　method　introduced　here　provides　a　rapid　and　sensitive　tool　for　monitoring　red　tide　pollution.