Invasive fungal mastoiditis is a rare entity, seen almost entirely in immunocompromized patients. It has been reported primarily in patients with leukaemia and more recently with acquired immunodeficiency syndrome. A literature search revealed only a few reports in diabetic patients, in whom the invasive fungus was identified as mucormycosis in all cases. We report the first case in the English literature of invasive septate fungal mastoiditis in a diabetic patient with intact tympanic membranes.