The presence of Callitriche cophocarpa Sendtn. in the Netherlands has, until recently, always been doubtful. Anatomical investigation of the pericarp has yielded, in the meantime, valuable new characters for distinguishing C. cophocarpa from C. platycarpa Kütz. In ascertaining these characters, some herbarium specimens of Dubbeldam (1848) and Meppel (1930) have now with certainty been identified as C. cophocarpa. It is not known whether or not this diploid taxon (2n=lo) is still present in the Netherlands; the related tetraploid C. platycarpa is, however, very common there. The possibility of the presence in the Netherlands of the hybrid of these two species, known from Denmark, Switzerland and eastern France, is discussed. C. cophocarpa differs from C. platycarpa mainly by morphological and anatomical characters of the fruit, and by its chromosome numbers. Some of these characters are discussed.