  Motivated by debris disk studies, we investigate the gravitational
microlensing of background starlight by a planetesimal disk around a foreground
star. We use dynamical survival models to construct a plausible example of a
planetesimal disk and study its microlensing properties using established ideas
of microlensing by small bodies. When a solar-type source star passes behind a
planetesimal disk, the microlensing light curve may exhibit short-term,
low-amplitude residuals caused by planetesimals several orders of magnitude
below Earth mass. The minimum planetesimal mass probed depends on the
photometric sensitivity and the size of the source star, and is lower when the
planetesimal lens is located closer to us. Planetesimal lenses may be found
more nearby than stellar lenses because the steepness of the planetesimal mass
distribution changes how the microlensing signal depends on the lens/source
distance ratio. Microlensing searches for planetesimals require essentially
continuous monitoring programs that are already feasible and can potentially
set constraints on models of debris disks, the supposed extrasolar analogues of
Kuiper belts.
