In the course of a study of prawns infected by epicarid isopods, I observed some specimens of Spirontocaris lilljeborgi (Dan.) showing deformations of the carapax very similar to those caused by parasitic isopods (fig. 1). In this case, however, the swellings of the carapax were not caused by epicarids, but by parasitic copepods of the family Choniostomatidae, lodged in the branchial cavity of the prawn. After painstaking dissection and examination of the appendages, I believe that my material belongs to a new species of the genus Choniostoma, for which the name Ch. rotundatum is proposed. Hitherto, 2 species only have been referred to the genus Choniostoma: Ch. mirabile Hansen, and Ch. hanseni Giard & Bonnier. These two species have been described in a very satisfactory way by Hansen (1897). In comparing the literature on the subject, I discovered that Nierstrasz & Brender à Brandis (1930) created a new genus which they called Heptalobus, but which obviously is synonymous with Choniostoma. The opinion of Nierstrasz & Brender à Brandis that ”the systematic position of this organism is quite unknown. Its characters furnish no clue as to its relationships” (1930, p. 5) is easily understood in the light of the astonishing superficiality of their study, which was strictly limited to what could be seen through a hand-lens.