  We consider the signatures of a population of primordial black holes (PBHs)
in future observations of 21cm radiation from neutral hydrogen at high
redshift. We focus on PBHs in the mass range $5 \times 10^{10} kg \lesssim
M_{PBH} \lesssim 10^{14} kg$, which primarily influence the intergalactic
medium (IGM) by heating from direct Hawking radiation. Our computation takes
into account the black hole graybody factors and the detailed energy dependence
of photon and e+/- absorption by the IGM. We find that for black holes with
initial masses between $5 \times 10^{11} kg \lesssim M_{PBH} \lesssim 10^{14}
kg$, the signal mimics that of a decaying dark matter species. For black holes
in the range $5 \times 10^{10} kg \lesssim M_{PBH} \lesssim 5 \times 10^{11}
kg$, the late stages of evaporation produce a characteristic feature in the
21cm brightness temperature that provides a unique signature of the black hole
population. If no signal is observed, then 21cm observations will provide
significantly better constraints on PBHs in the mass range $5 \times 10^{10} kg
\lesssim M_{PBH} \lesssim 10^{12} kg$ than are currently available from the
diffuse $\gamma$-ray background.
