The effective action of a dual Dirac string is determined within a 4-d
imensional formulation of Dirac's extension of Electrodynamics supplem
ented by the dual London equation of superconductivity. We identify th
e effective action calculated for classical electromagnetic fields ind
uced by a dual Dirac string with the string action and evaluate the st
ring tension. Our string action differs from that found by Nambu.