AbstractThe motility and fine structure of the marine planktonic dinoflagellate Kofoidinium and members of other related genera have been investigated. Several types of shape change were found to occur: slow morphogenetic changes (which also occurred as restitution movements in response to injury), movements associated with the ingestion of food and the evacuation of wastes, and more rapid movements concerned with the capture of prey. All these movements seemed to be brought about by the contraction of refractile tracts within the cytoplasm, which were found to contain 2.3‐nm filaments, some with a complex striated appearance. This and other evidence suggests that these filaments, which have counterparts in many other protists, are not actin filament