  The degeneracy between the disk and the dark matter contribution to galaxy
rotation curves remains an important uncertainty in our understanding of disk
galaxies. Here we discuss a new method for breaking this degeneracy using
gravitational lensing by spiral galaxies, and apply this method to the spiral
lens B1600+434 as an example. The combined image and lens photometry
constraints allow models for B1600+434 with either a nearly singular dark
matter halo, or a halo with a sizable core. A maximum disk model is ruled out
with high confidence. Further information, such as the circular velocity of
this galaxy, will help break the degeneracies. Future studies of spiral galaxy
lenses will be able to determine the relative contribution of disk, bulge, and
halo to the mass in the inner parts of galaxies.
