  We report on the recovery of a Blue Horizontal Branch (BHB) population
belonging to the Sagittarius Dwarf Spheroidal Galaxy (Sgr). The sequence is
clearly identified in the (V, V-I) Color Magnitude Diagram (CMD) obtained for
about 500,000 stars in the region of the globular cluster M~54. The BHB
morphology is similar to the analogous sequence in M~54, but it is
unambiguously associated with Sgr since {\it(i)} it is detected well outside
the main body of the cluster, up to more than 5 tidal radii from the cluster
center and {\it(ii)} the BHB stars follow the radial distribution of the other
stellar populations of Sgr. This finding finally demonstrates that the Sgr
galaxy hosts a significant (of the order of $\sim$10%) old and metal-poor
stellar population ([Fe/H]\ltsima -1.3; age \gtsima 10 Gyr), similar to that of
its oldest clusters (M~54, Ter~8). We also show that the Sgr BHB sequence found
here is the counterpart of the analogous feature observed by Newberg et al.
(2002) in the Sgr Stream, in a field more than 80$\degr$ away from the center
of the galaxy.
