| Summary: | <i>Phalacroma rotundatum</i> is a rare cosmopolitan heterotrophic dinoflagellate. This species, included in the IOC-UNESCO Taxonomic Reference List of Harmful Microalgae, may be a diarrhetic shellfish poisoning (DSP) toxin vector, but little is known about its ecophysiology and behavior. A vertical net haul collected during the austral summer of 2018 in Reloncaví Sound (Chilean Patagonia) revealed an unusually abundant population of <i>P. rotundatum</i> and prompted intensive 24 h sampling on 16−17 January to study the cell cycle and feeding behavior of this species. Hydrographic measurements from a buoy revealed the local characteristic estuarine circulation, with a brackish surface layer (salinity 26−28) separated from saltier, colder bottom waters by a pycnocline at a depth modulated by the tidal regime. A high proportion of <i>P. rotundatum</i> cells were packed with digestive vacuoles (peak of 70% at 14:00), and phased cell division (<i>µ</i> = 0.46 d<sup>−1</sup>) occurred 3 h after sunrise. The division time (T<sub>D</sub>) was 2 h. This is the first cell cycle study of <i>P. rotundatum</i>. The results here disagree with those of previous field studies that considered asynchronous division in some <i>Dinophysis</i> species to be related to heterotrophic feeding. They also question the very specific prey requirements, <i>Tiarina fusus</i>, reported for <i>P. rotundatum</i> in northern Europe.
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