This work includes survey part and experimental part. Thesamples for the survey part were collected from different localities aswell as from broiler farms during the period from may 2008 to May2009. The samples were taken from chickens suffered fromrespiratory signs. The clinical signs and grow lesions were recorded.Samples for bacteriological isolation were taken and other specimensfor histopathological examination were taken from lungs, airsacs,heart, liver, intestine, spleen, kidneys and joints.The experimental part was done on 30 chicks of one day age ofCubb breed. These chicks were given the isolated strain 078, E. coliin a dose of 108 CFU by different routes at the 5th day of age. Thesechicks were sacrifice at the is” and 21 st day of age. The clinical signsand gross were recorded. Histopathological specimens were takenfrom the same organs in the survey part.The gross lesions on the liver were dark-red color perihepatitisand grayish necrotic areas. The heart showed thick and opaquepericardium. The intestine showed watery contents and congestion.The lungs revealed pneumonic areas. The airsacs were opaque andthick. The kidneys were swollen and congested. The joints wereswollen.Microscopically, the liver shoed contestion. Hemorrhage,degenerative changes, edema, necrosis, leukocytic infiltration and-Biochemical characterization and serological identification provedthat the three Mycoplasma species could be identified as M. gallinarum,M. gallisepticum and M. anatis. The highest recovery rate ofM. gallinarum was (75%) from trachea, followed by tracheal swabs(36.36%). M. gallisepticum, the isolation rate was high from lungs(76.92%), followed by egg yolk (33.33%) and trachea (25%). On theother hand, M. anatis could be isolated from egg yolk and lungs (66.67%and 23.08%, respectively).Acholeplasma species could be isolated only from tracheal swabs(63.64%).On the other hand, SPA results revealed that three egg samples outof one hundred and twenty were positive for M. anatis andM. gallisepticum antibodies two isolates related to M. anatis with anincidence rate (66.67%) while one isolates related to M. gallisepticumwith an incidence rate (33.33%).