  In the framework of an ongoing ACS@HST project (HST program #10586, PI:
Aloisi) we have obtained deep multi-color imaging of the very metal-poor Blue
Compact Dwarf galaxy IZw18. The data were acquired in time-series fashion to
allow the identification of Classical Cepheids (CCs). The main aim of this
project is to constrain both the distance and the Star Formation History of the
galaxy. However, as a byproduct these data also provide new insights into the
properties of CCs at very low metallicities. We have identified 24 candidate
CCs in IZw18. New theoretical pulsation models of CCs specifically for the low
metallicity of this primordial galaxy (Z=0.0004, Y=0.24) have been computed to
interpret our results.
