  We examine the global stability of an exponential stellar disk embedded in a
dark matter halo and constrain the mass-to-light ratio of observed disks in the
I-band, Y_I. Assuming only that the radial surface density distribution of
disks is exponential, we derive an analytic upper limit: Y_I <~ 2.4h, for a
Hubble constant of 100h~\kms\mpc^{-1}. Using N-body simulations we derive a
stability criterion significantly different from that of previous authors.
Using this criterion we argue that, almost independent of the concentration of
the halo, Y_I <~ 1.9h. We discuss this result in relation to other independent
determinations of Y_I, its limitations and its implications for theories of
disk formation and barred galaxies.
