4-(3,7-Dimethyl-4-oxo-4,5-dihydroisoxazolo[4,5-<i>d</i>]pyridazin-5-yl)benzenesulfonamide

The nine-membered fused-ring system of the title pyridazine derivative, C(13)H(12)N(4)O(4)S, is approximately planar (r.m.s. deviation 0.027 Å), and the benzene ring of the phenyl-sulfamide substituent is aligned at 43.5 (1)° to the fused-ring system. The amine group of the sulfonamide substituent forms an N-H⋯O hydrogen bond to the ketonic O atom of two neigboring mol-ecules to generate a chain running along the c axis.