A description of a new species of Diaptomidae "Diaptomus" negrensis is given with a diagnosis. The variation of this species is very little. It has been found in various lakes at the left margin of the Rio Negro (Amazonas) near the city of Manaus. A map shows the distribution. A possible relationship with "Diaptomus" alter Herbst, 1960 is discussed but a certain relationship can't be given.