  We report on Australia Telescope Compact Array (ATCA) HI observations carried
out in the direction of bilateral supernova remnants (SNRs) with associated
neutron stars: G296.5+10.0 and G320.4--1.2, in a search for the origin of such
morphology. From these studies we conclude that in the case of G296.5+10.0,
located far from the Galactic plane, the HI distribution has not influenced the
present morphology of the SNR. In the case of G320.4--1.2, evolving in a denser
medium, the combined action of the central pulsar, PSR B1509-58, with the
peculiar distribution of the surrounding medium, has determined the observed
characteristics of the SNR.
