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Biaxial experiments with cruciform specimens can be used to determine the influence of the biaxial stress states on the material behavior, but are rare in literature. In the present study the cyclic deformation and fatigue behavior of a austenitic CrMnNi TRIP steel was investigated under biaxial out-of-phase and biaxial in-phase loading with different phase angles Φ as well as uniaxial loading. The biaxial experiments were performed by using a servohydraulic tension-compression test rig and cruciform specimens. The results show similar fatigue lives under out-of-phase loading in comparison to in-phase loading with Φ = 0° and much higher fatigue lives under 180° in-phase loading than under all other stress states. The reason was not given by the martensitic phase transformation of the steel which causes secondary hardening during cyclic deformation. No significant dependence of the fatigue life on the martensite formation and the secondary hardening were found. Moreover, it seems that there is no linear correlation between the phase angle Φ and the martensite formation as well as the secondary hardening. As possible explanations for the fatigue lives under biaxial in-phase and out-of-phase loading the microcrack propagation directions and the hydrostatic stress component were discussed.