For three Prorocentrum species, P. minimum (Pavillard) Schiller, P. rhathymum sp. nov., and P. triestinum Schiller, we have found that both flagella emerge from a single large pore. This arrangement is contrary to previous observations which suggested they arose from separate pores. Prorocentrum triestinum has only one anterior pore, whereas P. minimum and P. rhathymum sp. nov. have an additional auxiliary pore. The pores in all three species are formed by a series of small plates. The left and right valves on each species can be identified by ornamentation unique to each valve. Prorocentrum rhathymum sp. nov., isolated from the plankton of Cinnamon Bay, Virgin Islands, is described.