  Gas in the interstellar matter is generally organized in filamentary
structures, which may be also relevant for a complementary explanation of the
dark matter in the Galactic halo. We examine the possibility that such
structures may act as gravitational microlenses on background sources. To this
purpose, we derive the general properties of a cylindrical lens and compare the
light curves produced by such microlensing events with those generated by
spherically symmetric clouds. We find that the establishment of the symmetry of
the lens through the sole analysis of the light curve may be problematic, while
the analysis of the astrometric shift of the centroid of the image can
discriminate between the two classes of clouds. On the basis of our analysis,
we find that only gas filaments with a very high density could be detectable.
Such clouds are unlikely to exist in a long-lived state. Therefore,
microlensing cannot discriminate on the existence and the relevance of gas
filaments in the Halo, which could well be present and escape detection by
ordinary microlensing surveys.
