  IZw18, a blue compact dwarf (BCD) galaxy with the 2nd lowest metallicity
measured in a star-forming object, has been observed with all three instruments
on board the Spitzer Space Telescope. We present the deepest 5-36 micron
mid-infrared (mid-IR) spectrum of this galaxy as yet obtained, as well as 3.6
to 70 micron imaging results. As with SBS0335-052E, another BCD with similar
metallicity, IZw18 shows no detectable emission from polycyclic aromatic
hydrocarbons (PAHs). However, the continuum emission, from 15 to 70 micron, of
IZw18 has a much steeper slope, more characteristic of a typical starburst
galaxy of solar abundance. The neon abundance as measured from the infrared
fine-structure lines is ~1/23 Z_sun, and the sulfur abundance is ~ 1/35 Z_sun,
generally consistent with the nebular oxygen abundance of 1/30 Z_sun derived
from optical lines. This suggests that the extinction to the infrared emitting
regions of this galaxy is low, also in agreement with the optical Balmer line
ratios.
