Pulmonary infection caused by Arachnia propionica in a patient with hairy cell leukaemia is described. The organism is highly sensitive to penicillin but the patient succumbed to his illness despite treatment with penicillin and various other antibiotics. Various types of opportunist infections are common in patients with hairy cell leukaemia but we believe this to be the first report of infection by Arachnia propionica in such a patient.