ABSTRACT: Protoperidinium pellucidum is a pallium feeding heterotrophic dinoflagellate that captures phytoplankton cells individually and digests them externally. In laboratory cultures, P. pellucidum feeds on a variety of diatom species and a limitednumber of dinoflagellate species, and grows more rapidly on diatoms than dinoflagellates. When offered food in mixed assemblages, it feeds selectively on diatoms over dinoflagellates, and selects between diatom species. Selectivity between differentdiatom species does not appear to be related to size, and size alone does not explain the low selectivity for dinoflagellates. Computerized motion analysis studies of swimming behavior reveal that P. pellucidum appears to use chemoreception as themajor sensory mode to detect and locate food. When P. pellucidum passes near a food cell it circles around the cell several times before attaching to the food particle, apparently using chemoreception to judge the location of the cell. Detailedbehavioral observations reveal that P. pellucidum sometimes loses contact with motile dinoflagellate cells before the capture occurs; such losses were not observed with diatoms. In addition, motile dinoflagellate prey often escape after initialcapture, their swimming behavior causing the capture filament to break before the cell can be engulfed by the pallium of P. pellucidum; loss of a diatom after attachment was extremely rare. Feeding selectivity may be explained in part by the natureof the chemosensory signals given off by different prey types, and therefore the distance at which P. pellucidum can detect food, and in part by the lower capture success of P. pellucidum with motile prey.