The mean mass density of the universe is a primary target of observational cosmology. We devise a method to estimate this quantity by extending to spiral disks the argument that the peculiar motions of galaxies are generated by the overdensities of matter related to galaxy clustering. We obtain the density excess associated with the dark halos of spiral galaxies by considering, rather than the motion of a companion galaxy bound in an unknown orbit (as in earlier work of Davis & Peebles), the motion of test bodies rotating in the disk plane of spiral galaxies. This is done by means of a proper visible/dark mass decomposition of galaxy rotation curves. On scales R approximately 3-50 kpc (for a Hubble constant value of 50 km s-1 Mpc-1), we find that the mass excess around galaxies scales with R like the excess in number of spiral galaxies. On the assumption that light traces mass, we estimate the value of the cosmological density parameter to be OMEGA0 = 0.35 +/- 0.15. If the universe is flat, galaxy clustering is enhanced over matter clustering by a biasing factor of about 3.
