Summary Grazing of heterotrophic nanoflagellates on marine picophytoplankton presents a major mortality factor for this important group of primary producers. However, little is known of the selectivity of the grazing process, often merely being thought of as a general feature of cell size and motility. In this study, we tested grazing of two heterotrophic nanoflagellates, Paraphysomonas imperforata and Pteridomonas danica , on...