  Using an optical interferomertric method, the homogeneous cavitation density
of superfluid helium at $T=0.96\,$K is measured and found to be
$\rho_{cav}=0.1338\pm0.0002\rm\,g.cm^{-3}$. A well established equation of
state for liquid helium at negative pressures converts this to the cavitation
pressure $P_{cav}=-5.1\pm0.1\,$bar. This cavitation pressure is consistent with
a model taking into account the presence of quantized vortices, but disagrees
with previously published experimental values of $P_{cav}$.
