Addresses a practical problem arising in the calibration of bottom-lock Doppler sonar for the navigation of underwater robot vehicles. Employing a least-squares method, the rotational alignment offset between a bottom-lock Doppler sonar and a north-seeking gyroscope can be experimentally determined using sensors commonly deployed with a vehicle in the field. It requires sensor values from the vehicle's Doppler sonar and 3-axis gyroscope, and absolute vehicle position fixes from a long-baseline or short-baseline acoustic navigation system. The performance of the calibration method is evaluated with simulated Doppler data possessing measurement noise typical of that found in actual in-water vehicle sensor data.