  The damped Lyman alpha absorbers (DLAs) in quasar spectra are believed to be
the progenitors of present-day disk galaxies. We examine the probability for
microlensing of background quasars by stars in their DLAs. Microlensing by an
individual star should magnify the continuum but not the broad emission lines
of the quasars. Consequently, the equivalent width distribution of microlensed
quasars would be distorted. We model a representative spiral galaxy as a closed
system composed of a bulge, a disk, and a halo, and evolve the mass fraction of
stars in the disk based on the observed metallicity of DLAs at high redshifts.
The microlensing signatures are stronger if the halo of the galaxy is made of
Massive Compact Halo Objects (MACHOs). In this case, the distortion imprinted
by microlensing on the equivalent width distribution of quasar emission lines
can be detected with high significance in a sample of about 10 DLAs with HI
column densities N>10^21 cm^-2 and absorption redshifts z_abs<1. About a tenth
of all quasars with DLAs (N>10^20 cm^-2) might show excess variability on
timescales shorter than five years. A search for these signals would complement
microlensing searches in local galaxies and calibrate the MACHO mass fraction
in galactic halos at high redshifts.
