  Photometry with HST's ACS reveals that the subgiant branch (SGB) of the
globular cluster NGC 1851 splits into two well-defined branches. If the split
is due only to an age effect, the two SGBs would imply two star formation
episodes separated by $\sim$ 1 Gyr. We discuss other anomalies in NGC 1851
which could be interpreted in terms of a double stellar population. Finally, we
compare the case of NGC 1851 with the other two globulars known to host
multiple stellar populations, and show that all three clusters differ in
several important respects.
