The efficacy of Danofloxacin in treatment and prophylaxis of experimentally infected chicks was tested. One hundred Hubbard chicks were divided into five groups of twenty each. First group was infected at 15 days old with virulent strain of Mycoplasma gallisepticum (MG) S-6 then, treated with Danofloxacin (50ppm) one day after the appearance of the symptoms. Second group took a prophylactic dose for three days then, it was challenged withS-6, followed by treatment after 24 hours from the appearance of symptoms. Third group was dosed with Danofloxacin prophylactically without infection for 7 days and considered as residue group. Fourth group was used as negative control (uninfected, untreated). Fifth group was positive-control (infected, untreated). The results revealed that the second group (prophylactic then infected) gave more body weight and internal organs weight gain than the first (Infected then treated) and third (residue) groups which both gave nearly same results. Clinical symptoms were severe in the fifth and first groups accompanied with high mortality rate (80, 25%), respectively. Re-Isolation of M.gallisepticum was successful in first (infected, treated), second (infected, prophylactic) and fifth (positive-control) groups. Haemagglutination-inhibition (HI) titer was high in the first group during the first week after treatment, then, lowered by the second week. Meanwhile, HI titer of the second group was lower in first and second weeks after treatment than the first group, while, the residue and negative-control groups (3,4) showed a very high titer by the second week.