Respiratory manifestations in chickens which not respond to antibiotics in poultry farms with dampness, overcrowding and high temperature; environmental pollution with fungi was suspected. Organs (lung and trachea from 200 chickens) cultured on SDA media revealed different fungal causes. The macroscopic and microscopic examination of isolated fungi revealed 3 genera (104 samples for genus Aspergillus, 19 samples for Rhizopus and 15 samples for mucor). Antifungal sensitivity test for fungal isolates revealed that Aspergillus was highly sensitive for ketoconazole while Rhizopus and Mucor was highly sensitive for Amphotericin B. Experimental infection was carried out at one day old, 145 chicks were inoculated intranasally with 0.5 ml (1x 107) viable spores of fungal isolates . Clinical signs in form of severe respiratory signs were observed and the mortality rate was ranged from (1560%) which decreased to 0 up to 7% after treatment. Body performance showed marked decrease in body weight and body gain of infected chicken when compared with control. Also, liver and kidney functions revealed significant increase in the activities of ALT, AST, ALP, uric acid and creatinine, while showed significant decrease in the values of total serum protein, Albumin and globulin in infected chicks in comparison with control.