  We apply a unique gas fraction estimator to published X-ray cluster
properties and we compare the derived gas fractions of observed clusters to
simulated ones. The observations are consistent with a universal gas fraction
of 0.15+/-0.01h_{50}^{-3/2} for the low redshift clusters that meet our
selection criteria. The fair sampling hypothesis states that all clusters
should have a universal constant gas fraction for all times. Consequently, any
apparent evolution would most likely be explained by an incorrect assumption
for the angular-diameter distance relation. We show that the high redshift
cluster data is consistent with this hypothesis for \Omega_0<0.63 (95% formal
confidence, flat $\Lambda$ model) or \Omega_0<0.60 (95% formal confidence,
hyperbolic open model). The maximum likelihood occurs at \Omega_0=0.2 for a
spatially flat cosmological constant model.
