Two isomers were isolated during the reaction between 3-methyl-quinoxalin-2-one and bis-(2-chloro-ethyl)amine hydro-chloride. The crystal structure of one isomer has already been reported [Caleb, Bouhfid, Essassi & El Ammari (2009). Acta Cryst. E65, o2024-o2025], while that of the second isomer is the subject of this work. The title compound, C(14)H(15)N(3)O(3), has a new structure containing oxazolidine and quinoxaline rings linked by an eth-oxy group. The main difference between the two isomers is the position of the oxazolidine group with respect to the quinoxaline system. The dihedral angle between the fused planar rings and the oxazolidin-2-one ring is 41.63 (8)° in the title mol-ecule.