Folliculina simplex can attach itself to the shells of large gastropod larvae and is found as an epizoite on the veligers of Tonna galea, Cymatium nicobaricum, Charonia variegata, Cymatium parthenopeum, and several other Cymatiidae. Among all the planktonic samples taken throughout the North Atlantic, 12.6% included prosobranch veligers bearing folliculinid ciliates. Gastropod larvae, upon which F. simplex are found, originate on the tropical and warm-temperate continental shelf and they provide a means of long distance transport for the attached protozoans. Detailed examination of the lorica, using the scanning electron microscope, reveals that there are no distinct differences between F. simplex found upon veligers taken in the eastern and western Atlantic Ocean. Some species of gastropod veligers are preferred over others as a surface for attachment. Thus T. galea has the largest percentage of individuals bearing Folliculinidae - 45.3% among C. parthenopeum, only 4.2% bear folliculinids. However, the importance of each gastropod species to the dispersal of F. simplex depends not only upon the percentage of its larvae that bear the Protozoa but also on the absolute abundance of its veligers in the open sea. T. galea contributes 30.2% of the total larvae that bear folliculinids, but C. parthenopeum, because its veligers far outnumber those of any other species, contributes an unexpected 19.4%. The genetic and evolutionary consequences of the widespread dispersal of F. simplex are not yet evident, as nothing is known of the sexual processes (i.e. conjugation) among the folliculinid Protozoa.