The morphodynamics ofProrocentrumspecies have been studied comparatively in cultures and in plankton. Plankton populations show little morphological variation. In cultures a variety of forms, often similar to other species ofProrocentrumwhich have been described from distant seas, are found. Morphological variation inProrocentrumspecies depends upon various life phases, individual features of the clones, and various types of the cysts. Formation of morphological aberrants is also affected by aging of cultures. All protoplasmatic structures, including the cell membrane, possess their own particular features, changing in the course of the life cycle. Since both physiological and morphodynamic metabolisms are inseparable in their activities, it is useful to complete taxonomic diagnoses with physiological observations. Five new species are described:Prorocentrum cordiformis, Prorocentrum pomoideum, Prorocentrum pyrenoideum, Prorocentrum redfieldi, andProrocentrum levanlinoides. Morphodynamics inProrocentrum micansandProrocentrum scutellumwas also studied. Most of the observations were made in vivo. Specific morphological differences in form and structure of protoplasmatic organelles were demonstrated in each species concerned. An attempt was made also to base taxonomic diagnoses upon the apical tooth, trichocysts, and membrane structure.