  The oxygen abundance of the outer disk of the nearby S0 galaxy NGC 404, a
prototypical early-type galaxy with extended star formation, has been derived
from the analysis of HII region spectra. The high mean value found,
12+log(O/H)=8.6 \pm 0.1, equivalent to approximately 80% of the solar value,
argues against both the previously proposed cold accretion and recent merger
scenarios as viable mechanisms for the assembly of the star-forming gas. The
combination of the present-day gas metallicity with the published star
formation history of this galaxy favors a model in which the recent star
forming activity represents the declining tail of the original one.
