Source localization is a fundamental problem in seismology. Current localization techniques often rely on homogeneous isotropic models, even if a survey region is known to be geologically complex or anisotropic. We investigated a model's influence on localization, using data from a hydraulic injection experiment at the Continental Deep Drilling (Kontinentale Tiefbohrung, or KTB) site in Germany. We performed the localization with a grid-search algorithm and two additional methods for verification. From previous work, a homogeneous isotropic model based on a borehole check shot and a heterogeneous 3D isotropic model were available. Vertical seismic profiling (VSP) and borehole data as well as laboratory data from rock samples provided homogeneous anisotropic models. Although localization with the isotropic and anisotropic models led to the same magnitude of residuals and therefore to a comparable quality of fit, the distribution of the event cloud differed significantly for isotropic and anisotropic media. The isotropic model leads to a southward lateral shift of the event cloud of about from the injection point; the cloud appears almost centered at the injection point for the anisotropic models. This implies the classical diffusion concept for event generation, whereas isotropic models suggest the existence of a fault system providing a migration path for injected fluid, generating seismicity at an offset from the injection point. These results indicate that meticulous model building is essential for seismic-event localization and subsequent interpretation.