The diatom genus Cerataulina is represented by only four taxonomically accepted species of marine, truly planktonic affinity. Although its species may not be so frequent or abundant, one species, Cerataulina pelagica, may be abundant in coastal zones and may form harmful blooms in some parts of the world. Based on material collected by net during various oceanographic cruises and other surveys, along coasts of the Mexican Pacific, especially the tropical portion, marine phytoplankton samples were analyzed, from which two species of Cerataulina, namely Cerataulina dentata and C. pelagica, were studied by LM and SEM. Cerataulina dentata is a new record in the area. Observations made here confirm previous descriptions including various morphological details such as the shape of cells and chains, connections between sibling valves, valve face areolation, the projections of the valve poles which are costate ocelli, which may be winged and bearing a spine at the end, a single rimoportula, centrally (or subcentrally) or marginally located (depending of the species), and the structure of the copulae of the cingulum. Cerataulina pelagica is widely distributed whereas C. dentata is more restricted to tropical areas. Finally, morphological and distributional differences are discussed for the main three species of the genus.