The current work was carried out to clear-up the probable causes responsible for signs of infraorbital sinusitis with embryonic deaths of the fertile eggs in a multiple-age turkey's farm located in El-Minia Governorate, Upper Egypt. Clinical and necropsy examinations, and serological testing in association with conventional culturing procedures and molecular diagnosis (PCR) indicated that Mycoplasma gallisepticum (MG), either alone or coupled with other pathogens (Pasteurella multocida and Escherichia coli) was the predominant etiologic agent responsible for the respiratory problems including infraorbital sinusitis of the infected turkey flocks. The presence of MG infection in upper and lower respiratory tracts facilitates the portal entry of Pasteurella multocida infection to invade the lower respiratory tract and distributed systematically. This may increased the severity of the respiratory signs and increased the shedding rate of the in-ovo transmission of MG infection, which reduced the hatchability rate with dramatic embryonic death of the fertile eggs. Experimentally, in-ovo infection in fertile turkey's eggs with an MG-isolate isolated from the examined fertile eggs and organs of the naturally infected flock was carried out. Gross lesions and the histopathological alterations of the inoculated embryos were monitored during the period of experiment. Eight-days post inoculation, 28 % of the inoculated fertile eggs were succumbed. Diffuse edematous swellings and signs of dwarfisms particularly shortening legs (micromelia), in association with hyper-thickening of the membranes were obviously remarked in the inoculated embryos. The thickening of membranes was increased by increasing the days of incubation. Histopathological examinations of the experimentally inoculated embryos with MG (on the 3^rd, 6^th and 8^th days post-inoculation) were described in detail