The Author, in addition to the previous researches on the genus Oithona of the channel of Messina, studiesOithona helgolandicaand concludes thatOithona similismust be considered as synonym of the species by him studied. He demonstrates that the specimen hitherto described as male ofOithona helgolandicabelongs to another species for it presents notable morphological differences as to the female. In the gatherings of Messina the A. finds together mature females, some of which bringing attached the two spermatophorcs, a number of males, whose morphological characters agree perfectly with those of females, except for the genital segment and for the fusion of the fourt and fifth abdominal segment, instead of the first and the second, characteristic of the females.