1. The chain of evidence incriminating the house fly as a disseminator of typhoid fever is at present fairly complete, but many of the links are weak and not thoroughly strengthened by experimentation.2. The bulk of experimental work has hitherto been done under highly unnatural and artificial circumstances and the results so obtained cannot be accepted unreservedly as giving a correct view of conditions in nature.3. The experiments described in the present paper show that flies can ingest typhoid bacilli from natural matter,i.e.human faeces and urine, and carry them for a certain period of time.4. There is no evidence to show that the typhoid bacilli multiply in the house fly. On the contrary the evidence goes to show that they are not adapted for prolonged life on or in the fly.5. It thus follows that the house fly is a purely mechanical carrier of the typhoid bacillus and is not a natural “host” in the strict sense of the term.6. Many bacilli closely resemblingB. typhosusin cultural characteristics appear to be natural or, at least, common inhabitants of the intestine of the house fly. These are extremely likely to be mistaken forB. typhosusunless the most stringent tests are employed.7. As might be expected there is evidence to show that a process of bacterial selection occurs in the fly's intestine. Some bacteria appear to flourish but others are rapidly eliminated. Among the latter must be numberedB. typhosus.I have to thank my colleagues at the Lister Institute, particularly Dr Ledingham, Dr Henderson Smith and Dr Petrie, for much valuable help and advice during the course of these investigations. To Prof. Bainbridge, Dr Arkwright, Dr Penfold and Dr Macalister my thanks are also due, inasmuch as they helped to smooth over many difficulties both theoretical and practical.