While studying the plankton community of the “Loenerveense Polder”, one of the lakes along the eastern bank of the river Vecht, I came across an unusually large form of the genus Hyalosphenia. The species occurred in a qualitative net sample taken from the open water. Only one living specimen and two empty thecae were then observed. Though numerous net samples were taken from the same locality throughout the year, I did not meet with the animal again, only empty thecae sometimes being found. However when for the purpose of recovering more specimens, samples of bottom mud were taken, the living animal was seen again, though only once, and in one living specimen. After that I did not succeed in catching the animal again. Describing a new species from such scanty material as two living specimens and a few empty thecae may seem to be premature and not justified, but as the characteristic features are so unlike those of other species known of the genus Hyalosphenia, I feel sure that a mistake is out of the question.