The synthesis, separation, and structure determination of 6- and 7-acetyl--3-methyl-2-quinoxalinecarboxamide 1,4-dioxides are reported together with a comparison of their antibacterial activity. The structural assignment of these 6- and7-acetyl isomers was based on NMR analysis of related mono-N-oxide derivatives, which were obtained by treatment of the quinoxaline 1,4-dioxides with acetic anhydride--acetic acid or trimethyl phosphite. The compounds were screened for in vitro and in vivo activity against Escherichia coli, Salmonella choleraesuis, Pasteurella multocida, and Streptococcus pyogenes. Although the isomers were found to possess similar activity, the 7-acetyl isomer was more active therapeutically in mice than the 6-acetyl isomer when administered parenterally.