If heavy magnetic monopoles exist, then extreme magnetically charged black holes ($P^2=M^2$) with masses as small as 150${\left[\right({10}^{17}\mathrm{G}\mathrm{e}\mathrm{V})/\mathrm{\mu }]}^2$ g will be stable against evaporation over periods of ${10}^{10}$ yr or longer, where $\mu$ is the mass of the lightest monopole. It appears unlikely that such black holes exist in the present universe, but this conclusion depends strongly on the details of the first ${10}^{-24\mathrm{}}$ sec of the universe. Such long-lived small holes will exist in the late universe if there is at least 1 monopole per ${10}^{50}$${\left[\right({10}^{17}\mathrm{G}\mathrm{e}\mathrm{V})/\mathrm{\mu }]}^6$ baryons in the universe.

• Received 13 December 1982

DOI:https://doi.org/10.1103/PhysRevLett.50.1734