The mol-ecule of the title compound, C(10)H(10)N(2)O(2)S, is essentially planar, except for the ethyl group, which is twisted away from the carboxyl plane by -90.5 (3)°. In the crystal structure, mol-ecules are linked into a zigzag sheet propagating along the b axis by inter-molecular N-H⋯O and N-H⋯N hydrogen bonds.