A case of brainstem infection by Listeria monocytogenes is described. The patient was a 63 year old man previously in good health and his illness did not follow the usual bi-phasic pattern. There was no prodromal phase, and the progressive brainstem signs with a lymphocytosis and a normal sugar level in the CSF led to a tentative diagnosis of viral brainstem encephalitis. Ampicillin was begun only when signs of pulmonary infection developed. Clinical diagnosis is difficult but ampicillin should probably be used in any doubtful case in which a "viral" brainstem encephalitis is being considered.