The current disclosure generally relates to the field of saxitoxins and the identification of microorganisms capable of producing them. In particular, the saxitoxin A (sxtA) gene comprising catalytic domain sequences saxitoxin A1 (sxtA1) and saxitoxin A4 (sxtA4) is identified in a number of dinoflagellate species. The disclosure relates to methods of detection of saxitoxin-producing dinoflagellates by amplification and detection of the sxtA gene (in particular by PCR) and kits and primers for use in the method are also disclosed. Saxitoxin-producing dinoflagellate genera detected by the method include Alexandrium, Pyrodinium or Gymnodinium.