  We report results from near-infrared imaging of IZw18, the most metal-poor
galaxy in the local universe with NICMOS on board the Hubble Space Telescope
(HST). Observations were obtained in the broad F205W filter ($\sim$ K) and in
the medium-wide bands F171M and F180M, the latter which cover strong molecular
absorption features in cool stars. The new data, together with previously
obtained HST/NICMOS images in the F110W ($\sim$ J) and F160W ($\sim$ H)
filters, provide a census of the cool stellar population in this chemically
unevolved galaxy. We find that stars as old as $\sim 1$ Gyr are required to
explain the observed colour-magnitude diagrams. Combining broad and medium-wide
band photometry, we have classified the observed stars. The observed stellar
populations, to the depth of our data, are dominated by luminous red supergiant
stars. However, an intermediate-age component is also present. We have
identified carbon star candidates, and show that they dominate the stellar
content for intermediate mass and age. We show that low metallicity
intermediate-age stars have, for a given colour, a F110W ($\sim$ J-band) excess
compared to local carbon stars. The carbon stars identified in IZw18 are the
most distant (more than 10 Mpc) resolved examples yet discovered.
