Algeria has recently witnessed an unprecedented spread out of viral infections, among which Gumboro disease (IBD) ; Clinically characterized by dullness, depression, anorexia, white watery diarrhoea, ruffled feathers, soiled vent, severe prostration, vent picking and subnormal temperature followed by death ;Twenty-two 22 to 29 days-old broilers clinically suffering from Gumboro disease (IBD) were investigated histopathologicaly in order to evaluate the mechanism of IBD outbreak in vaccinated broiler chickens. The histological aspects of the bursa were observed using an optical microscope after being passed through different phases; Formalin fixing, ethanol dehydrating, paraffin soaking, microtome cutting, and finally Haematoxylin and eosin staining. The Microscopic lesions of the cloacal bursa were mainly illustrated by severe depletion, degeneration and necrosis of lymphocytes in the medulla of many bursal follicles , accompanied by inflammation and fibrous connective proliferation with mild lymphocytic infiltration in the inter follicular areas.