We consider a single Dubins-like mobile robot traveling with a constant longitudinal speed in a planar region supporting an unknown field distribution. A single sensor provides the distribution value at the current vehicle location. We present a new sliding mode control method for tracking environmental level sets: the controller drives the vehicle to the set where the distribution assumes a pre-specified value and ensures that the vehicle circulates along this set afterwards. The proposed control algorithm does not employ gradient estimation and is non-demanding with respect to both computation and motion. Its mathematically rigorous analysis and justification are provided. The effectiveness of the proposed guidance law is confirmed by illustrative examples and computer simulations.