  Deep long-slit spectroscopic observations of the blue compact galaxy IZw 18
obtained with the CFH 3.6 m Telescope are presented. The very low value of
oxygen abundance previously reported is confirmed and a very homogeneous
abundance distribution is found (no variation larger than 0.05 dex) over the
whole ionized region. We concur with Tenorio-Tagle (1996) and Devost et al.
(1997) that the observed abundance level cannot result from the material
ejected by the stars formed in the current burst, and propose that the observed
metals were formed in a previous star formation episode. Metals ejected in the
current burst of star formation remain most probably hidden in a hot phase and
are undetectable using optical spectroscopy. We discuss different scenarios of
star formation in IZw 18. Combining various observational facts, for instance
the faint star formation rate observed in low surface brightness galaxies van
Zee et al. (1997), it is proposed that a low and continuous rate of star
formation occurring during quiescent phases between bursts could be a
significant source of metal enrichment of the interstellar medium.
