  Using the Hubble Space Telescope, we resolve the red giant branch in the halo
of the S0 galaxy NGC 3115. We measure magnitudes and $(V-I)$ colours for stars
down to 1.5 magnitudes below the tip of the red giant branch. From the
brightest stars we estimate a distance modulus $(m-M)_0=30.21 \pm 0.30$,
corresponding to a distance of $11.0 \pm 1.5$ Mpc. This is in excellent
agreement with the value $(m-M)_0=30.17 \pm 0.13$ determined from the planetary
nebula luminosity function. Our results rule out the shorter distance modulus
$(m-M)_0=29.65$ determined from surface brightness fluctuations. A histogram of
$(V-I)$ colours shows a clear bimodality, indicating the presence of two
distinct halo populations of roughly equal size. One has [Fe/H]$\sim -0.7$ and
one has [Fe/H]$\sim -1.3$. This is the most distant galaxy in which a
Population II halo has been resolved, and it is the first time a colour
bimodality has been observed among the halo stars of any early-type galaxy.
