The dinoflagellate Dissodinium pseudolunula is the most common and widespread ecto-parasite of copepod eggs in neritic waters. When the host is absent, the species survives with a distinctive pair of twin resting cysts described as Pyrocystis margalefii. Based on live samples, the formation of the twin resting cysts is illustrated here for the first time. The gymnodinioid infective cells did not form overwintering cysts under unfavourable conditions. These are formed inside the secondary lunate sporangium.