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A new marine woloszynskioid dinoflagellate Dactylodinium pterobelotum gen. et sp. nov., collected from a southern Vietnamese estuary, was described on the basis of LM, SEM, and TEM, and molecular phylogeny inferred from rDNA sequences. This species had the smallest number of amphiesmal vesicles (5 latitudinal series) in woloszynskioid dinoflagellates assigned to the Suessiaceae and Borghiellaceae. The eyespot was of type B, composed of osmiophilic globules and brick-like material, located in- and outside of the chloroplast respectively. An apical structure comprised a pair of elongate anterior vesicles ( PEV). A large peduncle was conspicuous, located in the sulcal extension in the epicone, and supported by a microtubular strand of ~140 microtubules. Ultrastructural features of trichocysts represent a novel type in the Dinophyceae, bearing lateral hairs besides anterior fibers. The molecular phylogeny based on partial LSU rDNA showed the species in a basal position in the family Suessiaceae; this indicates the eyespot type B and PEV of the Borghiellaceae are ancestral states of the eyespot comprising brick-like material (type E) and an elongate apical vesicle of the Suessiaceae. [ABSTRACT FROM AUTHOR]