The antibiotic norfloxacin recrystallizes from acetonitrile as a dihydrate with the norfloxacin molecule in a zwitterionic form, i.e. 1-ethyl-6-fluoro-1, 4-dihydro-4-oxo-7-(1-piperazin-4-io)-3-quinolinecarboxylate dihydrate, C(16)H(18)FN(3)O(3).2H(2)O.