  NGC 4449 is an irregular galaxy with a moderately high star formation
activity. The massive stars in NGC 4449 have given rise to many bright HII
regions, superbubbles, supergiant shells, and "chimney-like" radial filaments.
ROSAT X-ray observations of NGC 4449 have revealed four point-like sources and
a wide-spread diffuse emission. The spectral properties of the diffuse
component suggest that the emission originates from hot interstellar gas. We
have compared deep ground-based H-alpha images with the X-ray images of NGC
4449 to determine the relationship between the hot (10^6 K) and the warm (10^4
K) components of the interstellar gas. We have also used an archival Hubble
Space Telescope WFPC2 image of NGC 4449 taken through the F606W filter to
examine the massive stellar content of the X-ray-emission regions. We find that
hot interstellar gas exists in (1) active star forming regions, including the
giant H II region CM 16, (2) probable outflows from star forming regions, and
(3) the supergiant shell SGS2. The X-ray data have been used to derive the rms
electron density, mass, and thermal energy of the hot interior of SGS2. Finally
we discuss the origin of SGS2 and implications of the detection of diffuse
X-rays in irregular galaxies.
