The isotopic shift of the Hg II u3bb3984 line is examined in the spectra of a number of strong mercury-manganese stars; the shift cannot be explained by line blending with lines of other elements in these stars, but represents real isotopic variations. An astrophysical log gf value - 1.8 is derived for u3bb3984, based on the Hg I u3bb4358 line strength in CrB and HR 1800. Elemental and (in some cases) isotopic abundances of Hg are derived using a spectrum synthesis technique; overabundances of up to 5.4 dex over the solar system over the solar system mercury abundance are found; the maximum overabundances may be temperature dependent.Peer reviewed: YesNRC publication: Ye