The damped Ly \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 \sim 10 DLAs with HI column densities N \ga 10 ^ { 21 } ~ { } { cm ^ { -2 } } and absorption redshifts z _ { abs } \la 1 .
About a tenth of all quasars with DLAs ( N \ga 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 .