  We present photometry with the Advanced Camera for Surveys (ACS) on the
Hubble Space Telescope (HST) of stars in the Magellanic starburst galaxy NGC
4449. The galaxy has been imaged in the F435W (B), F555W (V) and F814W (I)
broad-band filters, and in the F658N (Halpha) narrow-band filter. Our
photometry includes ~300,000 objects in the (B, V) color-magnitude diagram
(CMD) down to V < 28, and ~400,000 objects in the (V, I) CMD, down to I < 27 .
A subsample of ~200,000 stars has been photometrized in all the three bands
simultaneously. The features observed in the CMDs imply a variety of stellar
ages up to at least 1 Gyr, and possibly as old as a Hubble time. The spatial
variation of the CMD morphology and of the red giant branch colors point toward
the presence of an age gradient: young and intermediate-age stars tend to be
concentrated toward the galactic center, while old stars are present
everywhere. The spatial variation in the average luminosity of carbon stars
suggests that there is not a strong metallicity gradient (< 0.2 dex). Also, we
detect an interesting resolved star cluster on the West side of the galaxy,
surrounded by a symmetric tidal or spiral feature consisting of young stars.
The positions of the stars in NGC 4449 younger than 10 Myr are strongly
correlated with the Halpha emission. We derive the distance of NGC 4449 from
the tip of the red giant branch to be D=3.82 \pm 0.27 Mpc. This result is in
agreement with the distance that we derive from the luminosity of the carbon
stars.
