  We present a color-magnitude diagram (CMD) for a field in the giant tidal
stream of the Andromeda galaxy (M31). These observations, taken with the
Advanced Camera for Surveys on the Hubble Space Telescope, are 50% complete at
V~30 mag, reaching 1 mag below the oldest main-sequence turnoff. Striking
similarities between the stream and a previous spheroid CMD imply they have
very similar age and metallicity distributions, but present something of an
enigma; we speculate on possible interpretations of this result, but note that
none are without problems. Distinct multiple turnoffs, as might be expected
from pulses of star formation caused by interaction with Andromeda, are not
apparent in the stream CMD. Star formation in both fields lasted about 6
billion years, building up to relatively high metallicities and being largely
complete 6 billion years ago. The close similarity of the spheroid and stream
suggests that both may have derived from the same event; it would be worth
exploring to what extent stars in these structures are the remnants of a disk
galaxy that interacted with M31, or even were disrupted from the M31 disk
itself by the interaction.
