Prorocentrum micans Ehrenberg, Prorocentrum gracile Schütt and Prorocentrum sigmoides Böhm are planktonic dinoflagellates present along the Mexican Pacific coast. These three species can be treated as a species complex because of similar shape and size. The taxonomy of the group has been based on cell form and size, the presence or absence of apical spines and the ornamentation of the main valves. A morphometric study of samples from 11 locations from the Mexican Pacific coast and from laboratory cultures is presented. The presence of a mucron (small tooth) in the antapical region of P. gracile and P. sigmoides morphotypes was recorded for the first time. Morphotypes were defined using as the main criteria the size of pores in relation to the size of the depressions on the cell’s surface, the presence of a mucron in the posterior end of the cell and the shape of the valve margins. Results show a clear distinction between the morphotype P. micans and the P. gracile–P. sigmoides. P. sigmoides is suggested to be a junior synonym of P. gracile as no convincing characters could distinguish these two morphotypes.