Modular Invariance as an Explanation for the Absence of Monopoles.

It is shown that modular invariance provides a natural explanation for the absence of monopoles when assumed to be a discrete gauge symmetry. It follows that monopoles can not be seen because it is always possible to find a suitable gauge-fixing in which they are not present. This result relies upon an easy to prove but non-trivial property of the modular group. A modular-invariant formulation for the hamiltonian of the electromagnetism is given. No monopole arises if independent modular transformations are allowed at each point in space-time where point-like charges are present.