In the dinoflagellate Prorocentrum micans Ehr., changing the culture medium (switching from Erdschreiber's to Provasoli's medium) provokes the appearance of individuals that are morphologically different from the normal vegetative forms. Observations and microspectrofluorimetric measurements made in vivo of the relative amounts of nuclear DNA show that these forms are sexual forms; unlike the situation in dino-flagellate species that are known to be sexual, the male and female gametes of P. micans do not fuse. Cells playing the role of isogametes and containing q DNA pair and form a fertilization tube by means of which a donor cell (♂) injects its nucleus into a recipient cell (♀). After conjugation, the zygote containing 2q DNA replicates and thereafter contains 4q DNA. Two successive meiotic divisions lead to the formation of a tetrad in which each nucleus contains q DNA. Cells released from the tetrad seem to be adapted to Provasoli's medium and vegetative divisions occur again. The characteristics of sexual reproduction in those dinoflagellates in which this phenomenon has been described are reviewed and discussed.

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