  Models in which dark matter consists entirely of primordial black holes
(PBHs) with masses around $10^{17}$ g are currently unconstrained. However, if
PBHs are a component of the Galactic dark matter density, they will inject a
large flux of energetic particles into the Galaxy as they radiate. Positrons
produced by these black holes will subsequently propagate throughout the Galaxy
and annihilate, contributing to the Galactic 511 keV line. Using measurements
of this line by the INTEGRAL satellite as a constraint on PBH positron
injection, we place new limits on PBH abundance in the mass range $10^{16} -
10^{17}$ g, ruling out models in which these PBHs constitute the entirety of
dark matter.
