The purpose of this paper is to prove the relation incε = Curl κ relating the elastic strain ε and the contortion tensor κ, related to the density tensor of mesoscopic dislocations. Here, the dislocations are given by a finite family of closed Lipschitz curves in Ω ⊂ R 3. Moreover the fields are singular at the dislocations, and in particular the strain is non square integrable. Moreover, the displacement fields shows a constant jump around each isolated dislocation loop. This relation is called after E. Kröner who first derived the same formula for smooth fields at the macroscale.