In hydrogeological research for ground water as well as in many contamination studies, the direction of the subhorizontal ground water flow is an important information influencing further decisions in exploitation of the ground water resources or in application of remediation measures. New construction of the logging tool for determining azimuth of the ground water flow in a single well and its laboratory and field testing is presented. The sophisticated logging tool ‐ combination of an electronic compass with a special circular photometric sensor detecting the time changes of the tracer plume in the measuring slot ‐ offers the information on direction of subhorizontal ground water flow in a single well for velocities ranging between 10‐6 to 10‐1 m/s. The diameter of the tool is 76 mm, length 1.12 m and weight 13 kg. The tool is operating together with current logging systems.