The ectodomain of the paramyxovirus haemagglutinin neuraminidase (HN) glycoprotein spike can be divided into two regions: a membrane-proximalp stalk-like structure and a terminal globular domain. The latter contains all the antibody recognition sites of the protein, as well as its receptor recognition and neuraminidase (NA) active sites. These two activities of the protein can be separated by monoclonal antibody functional inhibition studies and mutations in the 91obular domain. Herein, we show that mutation of several conserved residues in the stalk of the Newcastle disease virus HN protein markedly decrease its NA activity without a significant effect on receptor recognition. Thus, mutations in the stalk, distant from the NA active site in the globular domain, can also separate attachment and NA. These results add to an increasin9 body of evidence that the NA activity of this protein is dependent on an intact stalk structure.