Of the many insects which we now know to be dangerous menaces to the health of man, the house-fly, Musca domestica, has few (if any) equals, but the facts relating to its germ-transmitting capacity are unfortunately still very incomplete. In the case of those Anopheline mosquitos which transmit the parasites of malaria, and the tsetse-flies, which infect man and other animals with trypanosomes, we are in a position to estimate the inefficiency and loss of life directly attributable to them, for in each case we know most of the species directly responsible for the dissemination of these parasites. But in the case of the house-fly we have no conclusive proof that Bacillus typhosus, for instance, is carried by one, or more than one, species of Musca. Further, by the time the cases are diagnosed, the infected flies have either died or disappeared, and it is rarely possible to trace the infection to them. It is true, however, that many observers have recorded the finding of pathogenic bacteria in, and on, species of Musca, but the final proof of the identity of these bacteria has invariably been faulty. Yet in spite of these discrepancies there is no doubt, from everyday practical observation the world over, that one, and more than one, species of Musca regularly carry the bacilli of the enteric fever, cholera and dysentery groups, as well as many other pathogenic organisms, especially the virus of trachoma, from infected material and deposit them on food and the human body. In investigating outbreaks of infectious diseases the house-fly should always be considered as a possible vector of the causal organisms.