A Crypthecodinium cohnii-like dinoflagellate was found to prey on the unicellular red alga Porphyridium sp. The cytoplasm of the prey is ingested by myzocytosis within 10–30 s, and the contents of up to 20 Porphyridium cells can be taken up by one dinoflagellate. The feeding tube is retracted after each uptake process. Cytochalasin D disturbs the suction and the retraction of the feeding tube. Feeding behaviour depends on the light–dark regime: the dinoflagellates prey preferentially in the dark period and finish the trophic phase at the beginning of the light period. They then assemble and encyst, and digestion takes place in the light period. Cell division is restricted to the encysted stage. At the end of the light period excystment takes place, combined with defecation. Isogamy as well as anisogamy and nuclear cyclosis were observed. The trophonts have very thin thecal plates and a microtubular basket with two kinds of elongate vesicles. The microtubules of the tubular basket are also found in young cysts, formed after the end of the feeding period. The fine structure of the digestion vacuoles and of the faecal bodies is described.