  We present a simple metallicity estimator based on the logarithmic [NII]6584
/ Halpha ratio, hereafter N2, which we envisage will become very useful for
ranking galaxies in a metallicity sequence from redshift survey quality data
even for moderately low spectral resolution. We have calibrated the N2
estimator using a compilation of HII galaxies having accurate oxygen
abundances, plus photoionization models covering a wide range of abundances.
The comparison of models and observations indicates that both primary and
secondary nitrogen are important for the relevant range of metallicities. The
N2 estimator follows a linear relation with log(O/H) that holds for the whole
abundance range covered by the sample, from about 1/50th to twice the Solar
value (7.2 < 12+log(O/H) < 9.1). We suggest that the ([SII]6717,6731 / Halpha)
ratio (hereafter S2) can also be used as a rough metallicity indicator. Because
of its large scatter the S2 estimator will be useful only in systems with very
low metallicity, where [NII]6584 is not detected or in low resolution spectra
where [NII]6584 is blended with Halpha.
