Four broiler houses in Sharkia governorate were examined to studythe microclimatic condition, occurrence of airborne bacteria, their effectson broiler performance and occurrence of pathogenic bacteria in air indifferent seasons.1- Microclimatic conditions in broiler houses:1- There was an irreversible relationship between age of broilers and relative humidity, ammonia and carbon dioxide.2- Highest temperature was recorded in summer (34.4°C±0.41), spnng(27.3°C±O.35), autumn (26.3°C±O.36) and winter (22.7°C±O.82).3- Highest relative humidity was recorded in winter (81.5% ±22.6)followed by spring (75.2%±1.64), autumn (71.5%±2.27) and summer(7 1.4%± 1.84).-c-4- Highest ammonia concentration m (ppm) was recorded in winter(lO.7±1.65), spring (9.3±O.83), autumn (8.7±O.99) and summer(6.4±O.64).5- Highest carbon dioxide concentration in (celL) was reported in winter(3.78±0.42), followed by spring (2.92±O.22), summer (2.53±O.22) andautumn (2.23±O.3).11- Occurrence of airborne bacteria in broiler houses:1- There was a steady increase in the viable count of airborne bacteriathroughout the fattening period of broiler. The maximum bacterial