Observation of superdeformation in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi mathvariant="normal">Hg</mml:mi></mml:mrow><mml:mprescripts/><mml:mrow/><mml:mrow><mml:mn>191</mml:mn></mml:mrow><mml:mrow/><mml:mrow/></mml:mmultiscripts></mml:mrow></mml:math>

The first observation of superdeformation in the mass region A\ensuremath{\simeq}190 is reported. A rotational band of twelve transitions with an average energy spacing of 37 keV, an average moment of inertia ${\mathrm{scrF}}^{(2)}$ of 110 ${\ensuremath{\Elzxh}}^{2}$ ${\mathrm{MeV}}^{\mathrm{\ensuremath{-}}1}$, and an average quadrupole moment of 18\ifmmode\pm\else\textpm\fi{}3 e b has been observed in $^{191}\mathrm{Hg}$; this band persists at low rotational frequency. These results are in excellent agreement with a calculation that predicts an ellipsoidal axis ratio of 1.65:1 for the superdeformed shape in this nucleus.