  Using HST and ground-based optical and NIR data, we investigate whether the
blue compact dwarf (BCD) galaxy I Zw 18 has an extended low-surface-brightness
(LSB) older stellar population underlying the star-forming regions, as is the
case in evolved iE/nE BCDs. Subtraction of narrow band H_alpha and [O III]
exposures from R and V images shows that the filamentary LSB envelope extending
out to ~2 kpc away from the starburst region, and hence the optical broad-band
colors observed therein, are due mainly to ionized gas emission. Ionized gas
accounts already at a galactocentric distance of 0.7 kpc for more than 80% of
the R band line-of-sight intensity and contributes more than 40% of the
integrated R band light of I Zw 18. The structural properties (such as the
exponential scale length) of the stellar LSB component underlying the extended
ionized gas emission place I Zw 18 among the most compact BCDs studied so far.
Contrary to evolved nE/iE BCDs, the stellar component in I Zw 18 shows no
appreciable color gradients over a range of ~8 mag in surface brightness.
