— The paper studies the observability properties of the relative localization of two Autonomous Underwater Vehicles (AUVs) equipped with depth sensors, linear/angular velocity sensors, and communication devices with range measurement. The conditions that ensure observability of the linearized model and locally weak observability of the nonlinear system are derived. An Extended Kalman Filter is then designed aimed at estimating the relative position between two AUVs. Simulations in 3D and reconstruction from experimental data in 2D provide a numerical validation of the analysis.