  Using the Australia Telescope Compact Array, we have carried out a survey of
the HI emission in the direction of the ``barrel-shaped'' supernova remnant
(SNR) G320.4-1.2 (MSH 15-52) and its associated young pulsar B1509-58. The
angular resolution of the data is 4.0x2.7 arcmin, and the rms noise of the
order of 30 mJy/beam (~0.5 K). The HI observations indicate that the N-NW radio
limb has encountered a dense HI filament (density ~12 cm^-3) at the same LSR
velocity than that of the SNR (V_LSR ~ -68 km/s). This HI concentration would
be responsible for the flattened shape of the NW lobe of G320.4-1.2, and for
the formation of the radio/optical/X-ray nebula RCW 89. The emission associated
with the bright knots in the interior of RCW 89 can be explained as arising
from the interaction between the collimated relativistic outflow from the
pulsar and the denser part of this HI filament (density ~15 cm^-3). The S-SE
half of the SNR, on the other hand, seems to have rapidly expanded across a
lower density enviroment (density ~0.4 cm^-3). The HI data also reveal an
unusual HI feature aligned with a collimated outflow generated by the pulsar,
suggestive of association with the SNR. The anomalous kinematical velocity of
this feature (V_LSR ~ 15 km/s), however, is difficult to explain.
