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In this paper I will discuss the possibility of a reionization of theUniverse due to the photons emitted by evaporating primordial black holes(PBHs); this process should happen during the last stages of the PBHs life,when the particle emission is very intense. I will study the time evolution ofthe ionization degree x, of the plasma temperature $T_{e}$ and of the photonnumber density $n_{\gamma}$ characterizing the Universe after the recombinationepoch: a system of coupled differential equations for these variables is solvedin an analytical way by assuming, as a photon source, PBHs having an initialmass $M\sim 10^{14}$ g. The results I obtain prove that such a kind ofreionization is possible, being able to increase the ionization degree of theUniverse from a value x=0.002 (just after the recombination) to values near 1(when the black holes evaporation ends).Comment: Plain Tex, 6 pages. Contribution to the 12th Italian Conference on  General Relativity and Gravitational Physics. Figures on reques

REIONIZATION OF THE UNIVERSE INDUCED BY PRIMORDIAL BLACK HOLES