A relationship was established between the steering angle and the turning radius of an autonomous four-wheeled, rice transplanter travelling in a paddy field. The transplanter was operated at three different forward speeds in circular paths in a flooded paddy field. The vehicle position was measured using real-time kinematic global positioning system. Measured values were compared with theoretical values of turning radii obtained by a geometrical vehicle model. The predicted turning radius, however, was much smaller than the measured turning radius, the difference being attributed to poor lateral support of the tyres when operating on the soft top soil of a flooded paddy field. © 2000 Silsoe Research Institute.