Magnetic susceptibility of myelin induces morphology- and orientation-dependent perturbations of the B_0-field. At the microstructural scale of brain white matter, the main contribution to these perturbations comes from myelin. Here, we show that a consequence of this is a fiber-orientation-dependent bias of the DWI-signal across white matter regions in an ex vivo rat brain. This implies that biophysical modeling must regard susceptibility at the microstructural scale in order not to become biased as well. Higher field strength will increase the bias and vice versa. Hence, especially pre-clinical scans are affected.