COS spectra of I Zw 18. The Lyα absorption (from I Zw 18, the Milky Way, and the high-velocity cloud) was removed and the flux was normalized to the local continuum. The numerous absorption features often come in pairs: the weaker − redshifted − absorption feature is from I Zw 18; the stronger feature arises in the Galaxy. A non-LTE, photospheric model spectrum (no winds) of the stellar population (10 Myr age with a continuous star formation hypothesis) and log   (Z / Z_⊙) = −1.7 is shown in blue. While many features such as C iiiλ1175 and the Si ivλ1400 doublet appear to be well matched by the stellar model, other high-ionization features such as He iiλ1640 and the C ivλ1549 doublet are not well matched. The origin of these lines (e.g., WR stars, H ii region...) is unclear.