The sexual resting cyst (hypnozygote) of the red-tide dinoflagellate Alexandrium minutum Halim (Dinophyceae), the type species of Alexandrium, is described from surface sediments collected from the Port River, South Australia. The clear, mucoid cysts were roughly hemispherical in shape, circular in outline when seen from above, and reniform when viewed from the side. This cyst type is distinct from the cylindrical cysts of A. tamarense (Lebour) Balech, and similar to the discoid cysts of A. hiranoi Kitaet Fukuyo and A. lusitanicum Balech.