The paramyxovirus family includes important human pathogens such as measles, mumps, respiratory syncytial virus, and the recently emerged, highly pathogenic Hendra and Nipah viruses. The viral fusion (F) protein plays critical roles in infection, promoting both the virus-cell membrane fusion events needed for viral entry as well as cell-cell fusion events leading to syncytia formation. We describe the surprising finding that addition of the short epitope HA tag to the cytoplasmic tail (CT) of the Hendra virus F protein leads to a significant increase in the extent of cell-cell membrane fusion. This increase was not due to alterations in surface expression, cleavage state, or association with lipid microdomains. Addition of a Myc tag of similar length did not alter Hendra F protein fusion activity, indicating that the observed stimulation was not solely a result of lengthening the CT. Three tyrosine residues within the HA tag were critical for the increase in the extent of fusion, suggesting C-terminal tyrosines may modulate Hendra fusion activity. The effects of addition of the HA tag varied with other fusion proteins, as parainfluenza virus 5 F-HA showed a decreased level of surface expression and no stimulation of fusion. These results indicate that additions to the C-terminal end of the F protein CT can modulate protein function in a sequence specific manner, reinforcing the need for careful analysis of epitope-tagged glycoproteins. In addition, our results implicate C-terminal tyrosine residues in the modulation of the membrane fusion reaction promoted by these viral glycoproteins.