Chlamydophila psittaci is considered one of the important bacterial agents which affecting poultry with zoonotic importance to human health. This study designed to investigate the antibacterial action of aqueous neem leaves extract (Azadirachta indica) against experimentally infected broiler chickens with chlamydia. Seventy, one day old Hubbard chicks were randomly selected and divided equally into seven groups, three of the infected groups were treated with neem extract at varied concentration of 4%, 6% and 8% in drinking water compared with the fourth that treated with oxytetracycline. Based on clinical and postmortem examination, growth performance, serum biochemical analysis and histopathological findings the results were evaluated. Chicks received 8% extract at 8 days old for five successive days and control showed normal level of alanine aminotransferase, aspartate aminotransferase, uric acid and creatinine 30 U/L, 26 U/L, 3.90 mg/dl and 0.67 mg/dl respectively, while treated infected groups revealed lower levels unlike infected untreated showed higher levels 68 U/L, 62 U/L, 5.20 mg/dl and 1.62 mg/dl respectively. Moreover treated groups with neem extract of 4%, 6% and 8% recorded significantly better body weights 1130.56 gm, 1135.70 gm and 1254.60 gm respectively, than infected untreated group 963.25 gm at 32 days old as well as feed conversion rate 2.26, 1.94 and 1.77 than 3.80 respectively, at 24 days old. Histopathological examination of infected group showed pulmonary inflammation, myocarditis and hepatic necrotic foci while confirmed that treatment with 8% neem extract resulted in complete recovery of lung tissue and normal myocardium. It is concluded that aqueous leaves neem extract especially 8% concertation had an excellent antichlamydial medication without side effects and recommended in the control of chicken chlamydiosis