Preliminary results from bidimensional long-slit spectroscopic observations of the prototypical low-metallicity dwarf galaxy I Zw 18 are presented. Eight evenly spaced slit positions were observed in order to cover the body of the galaxy and part of the ionized halo seen in deep H alpha images. The study of the ionization structure and the abundances of I Zw 18 has been performed in two dimensions for the first time. Chemical abundances of several elements have been obtained for up to sixteen positions over the galaxy. Our results confirm and extend previous suggestions that the galaxy presents a homogeneous chemical composition. The average abundances obtained are 12 + log(O/H) = 7.19 +/- 0.07, 12 + log(N/H) = 5.64 +/- 0.12 for oxygen and nitrogen respectively, and He/H = 0.073 +/- 0.004 for helium, equivalent to a mass fraction Y = 0.228 +/- 0.009. However, some underlying absorption has been detected in the central knots, that could be affecting the flux in the helium lines. When the bright central knots are excluded from the mean we derive Y = 0.242 +/- 0.010, a value two sigma higher than the abundance derived using the spectra of the central knots only.