We consider a single Dubins-like vehicle traveling with a constant longitudinal speed in a planar region supporting an unknown field distribution. A sensor provides the distribution value at the vehicle location. We present a new sliding mode control method for tracking environmental level sets: the vehicle is steered to the set where the distribution assumes a pre-specified value and 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 justification is provided. The effectiveness of the proposed guidance law is confirmed by computer simulations. (c) 2012 Elsevier Ltd. All rights reserved.