Chronic frontal sinusitis and osteolysis in a herd of desert bighorn sheep (Ovis canadensis nelsoni) in Zion National Park, Utah, was attributed to bacterial infection secondary to aberrant nasal bot migration. Analysis of skulls and live adult sheep (equal to 1 year old) showed that 9 to 22 animals were affected. One ram and 1 ewe were treated by trephination and irrigation. Treatment was successful for the ewe, but not for the ram. Radiography, thermography, and blood profiles were used in an attempt to find a way to diagnose sinusitis in its early stages; at this point, only thermography shows promise.