  We report spatially resolved 11.8 micron images, obtained at the W. M. Keck
10 m telescope, of the protoplanetary disk around the pre--main-sequence star
HK Tau B. The mid-infrared morphology and astrometry of HK Tau B with respect
to HK Tau A indicate that the flux observed in the mid-infrared from HK Tau B
has been scattered off the upper surface of its nearly edge-on disk. This is
the first example of a protoplanetary disk observed in scattered light at
mid-infrared wavelengths. Monte Carlo simulations of this disk show that the
extent (FWHM =0."5, or 70 AU) of the scattered light nebula in the mid-infrared
is very sensitive to the dust size distribution. The 11.8 micron measurement
can be best modelled by a dust grain population that contains grains on the
order of 1.5-3 micron in size; grain populations with exclusively sub-micron
grain sizes or power law size distributions that extend beyond 5 micron cannot
reproduce the observed morphology. These grains are significantly larger than
those expected in the ISM implying that grain growth has occurred; whether this
growth is a result of dust evolution within the disk itself or had originally
occurred within the dark cloud remains an open question.
