||This first part aims to study the correlation between the cyclic softening observed during low-cycle fatigue in a 2507 duplex stainless steel (DSS) alloyed with 0.17 % N and the dislocation structure development. Tension-compression and tension-compressiontorsion fatigue tests were carried out under total strain control at Δεteq /2 = 0.5 %. The dislocation structures were characterized by transmission electron microscopy during cycling with interrupted tests. The austenite showed planar arrangements of dislocations during all the fatigue life, the ferrite showed evolution from a homogeneous distribution of dislocations to a structure of dislocation bundles in tension-compression, and to a refined wall structure in tension-compression-torsion. It is proposed that the evolution of the dislocation structure in the ferrite plays an important role during the cyclic softening in the DSS.