  We present high spatial resolution observations of the blue compact dwarf
galaxy IZW18 performed in the Halpha line with a scanning Fabry-Perot
interferometer at the CFH telescope. Morphological structure of the galaxy in
Halpha and in the red continuum is investigated. We also analyse the velocity
field of the ionized gas. Besides the two compact HII components of the main
body we find a population of small HII regions in its surroundings whose
diameter distribution and Halpha luminosity function are consistent with those
observed in dwarf Irregular galaxies. In the main body of the galaxy besides of
the NW and SE red continuum peaks which are displaced with respect to the
Halpha maxima, three new red condensations have been discovered. They have no
clear Halpha counterparts. The velocity field in IZW18 shows peculiar motions
superimposed on a quite regular background implying solid-body rotation with a
gradient of about 70km/sec/ kpc. The Halpha line profiles exhibit an asymmetric
structure, except for the NW main compact component. At least part of this
asymmetry could result from accreted and/or expelled surrounding gas from the
main star-forming core(s) of the galaxy. Contrary to previous suggestions that
the south-west and north-east extensions of this galaxy are diffused emission
produced by bipolar emitting gas we provide evidence that they are HII regions
powered by star formation sites. The redshift of the Zwicky's "flare" has been
measured for the first time and corresponds to the same velocity as IZW18. In
such a context the optical ridge that appears to be an isolated morphological
structure has a shape that may result from the gravitational interaction with
the Zwicky's "flare" if this latter is a neighbour extreme dwarf object.
