The house rat is rarely found in Europe while in many parts of the tropics it is common and in close association with man. The fact that the sewer rat avoids the upper portions of houses probably explains the greater infrequency of plague epidemics in Europe where this rodent is common. In former ages when the house rat was prevalent in Europe we had great epidemics there. _Mus (Rattus) norvegicus_ is of stout build with a blunt nose and small opaque ears which barely reach the eyes when laid forward. The tail is shorter than the length of the head and body together (89% of such length). With _Mus rattus_ we have a delicately built rat with a slender head and sharp nose. The ears are translucent and large and reach beyond the middle of the eye when extended. The rather delicate tail is about 25% longer than the length of the head and body taken together. Recently it has been found that a guinea pig set free in a house suspected of having plague fleas becomes infected if such fleas are present. The fleas would probably prefer the guinea pig to man and such a measure would in some degree be protective to man. It is however for the detection of plague infection that the measure is employed and the guinea pig is termed the “Plague barometer.” In Madras there is practically an absence of _Mus norvegicus_ although _Mus rattus_ is present in numbers and the comparative freedom of the city from plague is striking. The principal rat flea of the Orient is _Xenopsylla cheopis_. This flea is without combs like _Pulex irritans_, the human flea, but is of a lighter color and has an ocular bristle near the upper margin of eye and two bristles posterior to the antennae. In Europe and the United States _Ceratophyllus fasciatus_ is the common rat flea. Many other species of fleas transmit plague and it is also possible that the bedbug may play a part in spreading infections from man to man. Vergbitski has transmitted plague from man to the rat by infected bedbugs. In Siberia, a marmot, the tarabagan, is supposed to play the part of the rat in plague transmission. In California, the ground squirrel, _Citellus beecheyi_, has become infected and may transmit the disease by its flea, _Ceratophyllus acutus_. In the Tropics plague tends to prevail only at times when the temperature is between 10° and 30°C. It is the effect on the flea of cold weather which causes the disappearance of bubonic plague at such times. The bacillus of plague can withstand freezing temperatures. Sunlight and drying are the especially inimical factors for _B. pestis_. Dry seasons are inimical to the spread of plague and it is especially in very rainy seasons that epidemics rage.