Where rats are on board ship fumigations with sulphur dioxide, carbon monoxide or funnel gases are usually employed. Hydrocyanic acid gas is a most efficient destroyer of rat and flea life. The great objection to its use is its danger to those using it in fumigation. Liston allows this gas, developed from ½ ounce KCN, to act for four hours in a space of 100 cubic feet. The great danger from the use of this gas in holds of ships is that it tends to collect in detached spaces or pockets and remains following ventilation of the hold so that persons entering such spaces suffer the poisonous effects of the gas. Some cargo ships have a rat-run built to extend fore and aft and leading to a receptacle in which the rats are caught. Rats naturally choose a tube or similar opening so they get into this little passageway which is so constructed that their return is obstructed. This scheme is used in setting traps, either covering the traps with hay and leaving a small opening or placing the trap under an inclined plank or placing it at the end of an iron or terracotta pipe. There is not much danger of rats getting aboard a ship lying out from the dock. It is when a ship goes alongside a dock that we can expect rats to come aboard. Phosphorus paste made up with a glucose base and containing about 4% of phosphorus is spread on pieces of stale bread, 1 inch square and ¾ inch thick. Whatever poison is used, whether strychnine, arsenic or phosphorus, it should be placed in boxes which have openings large enough to let the rats in but too small for domesticated animals. Barium carbonate is a useful rat poison. Cats will very rarely attack the fierce sewer rat. _Danysz virus._—Many workers, during plague outbreaks, have tried to exterminate rats by impregnating bread or other bait with bacterial cultures. The best known of these viruses, as they are called, is that of Danysz. The organism is closely related to _B. enteriditis_ of Gaertner and is supposed to bring about a fatal infection in the rats. As a matter of fact the cultures quickly cease to be virulent and their use has been generally abandoned. Simpson, however, thinks well of this measure and employed it with success in South Africa. He kept up the virulence of his cultures by frequent passage through animals. In rat extermination it is advisable to employ Heiser’s Manila plan. In this, the location in the city of the plague rats brought into the laboratory is noted and radiating lines made from such foci. Plague-infected rats are rarely found more than a few squares from the focus. The periphery of the infected area is then considered as an outer zone for the fight and the house-to-house extermination is carried on toward the center of the area. If, on the other hand, one should start at the center and work peripherally the infected rats might be spread all over the city.