  We calculate the magnification light curves due to stellar microlensing of
gamma-ray burst (GRB) afterglows. A GRB source appears on the sky as a thin
ring which expands faster than the speed of light and is maximally magnified as
it crosses the lens caustics. While a single star lens produces a single peak
in the magnification light curve, binary star lenses may produce multiple
peaks. The shape of the magnification light curve provides invaluable
information on the surface brightness distribution of the afterglow photosphere
on sub micro-arcsecond scales. We find that all afterglows are likely to show
variability at the level of a few percent about a year following the explosion,
due to stars which are separated by tens of Einstein radii from their
line-of-sight.
