Plankton samples from the Pacific coast of Mexico included some Pleurosigma species, of which three are particularly interesting: Pleurosigma ibericum Peragallo (typified here), P. distinguendum Hustedt, and a species fully matching all available data for Pleurosigma elongatum var. gracile Grunow. The latter diatom, here described as Pleurosigma gracilitatis sp. nov., cannot taxonomically be placed as a variety of P. elongatum W. Smith, as verified by a study of the type of the latter species. Its specimens were seen in various stages of valve morphogenesis. Type slide numbers are designated for Pleurosigma normanii Ralfs in Pritchard and P. rhombeum (Grunow in Cleve et Grunow) H. Peragallo.