  Recently, radial velocities have been measured for a large sample of M giants
from the 2MASS catalog, selected to be part of the Sgr dwarf leading and
trailing streams. Here we present a comparison of their kinematics to models of
the Sgr dwarf debris orbiting Galactic potentials, with halo components of
varying degrees of flattening and elongation. This comparison shows that the
portion of the trailing stream mapped so far is dynamically young and hence
does not provide very stringent constraints on the shape of the Galactic
dark-matter halo. The leading stream, however, contains slightly older debris,
and its kinematics provide for the first time direct evidence that the
dark-matter halo of the Galaxy has a prolate shape with an average density axis
ratio within the orbit of Sgr close to 5/3.
