It has been shown, in various parts of India, that the number of rat-fleas found on rats varies with the seasons. This seasonal variation of rat-fleas corresponds in a general way with the plague mortality. During the season when plague is bad the average number of fleas per rat is above the mean. During the non-epidemic season it is below the mean. The height of the epidemic corresponds fairly closely with the season of maximum flea prevalence. Nineteen species of _Pulicidæ_ are more or less habitually obtained by collectors on rats and mice. But the great majority of these may be called casual visitors. Six species of _Sarcopsyllidæ_ are also occasionally found on rats. These are the burrowing chigoes and their allies which usually attack the head and ears of rats. The species of flea commonly found on rats are five in number, and the readiness with which they bite human beings has been carefully studied. 1. _Xenopsylla cheopis._ This is the oriental rat-flea first described by Mr Charles Rothschild from specimens collected in Egypt. The true home of this flea appears to be the Nile valley, where it may be found in plenty on various hosts. Many of these are desert animals and the flea shows a preference for rodents. Having been distributed all over the world by rats, it now occurs, occasionally, in all warm climates. It is the common rat-flea of the tropical and sub-tropical world. In India it often happens that the whole of the fleas collected from rats prove to be of this species. But it cannot, apparently, flourish in cold countries. In the warmer temperate zones, such as the Mediterranean and Australian seaports, it occurs in varying proportions according to the time of year. The numbers decline with cold weather. It readily bites man and is more active than any other flea in the transmission of plague. For this reason it is sometimes spoken of as “the Plague-flea.” It is a smaller and a lighter coloured insect than the human flea. 2. _Ceratophyllus fasciatus._ This is the common European rat-flea. It is the rat-flea of the temperate as opposed to the hot countries of the world. It is commonly found on black and brown rats in the British Islands and the other countries of Northern and Central Europe. It readily bites man, and there is no reason to suppose that, other conditions being equal, it would not be as efficient an agent in spreading plague as the last species has been shown to be in India. 3. _Ceratophyllus anisus._ This is a closely allied species of rat-flea which replaces the last in China and Japan. 4. _Leptopsylla musculi._ This is the mouse-flea and it is as widely distributed over the globe as its host. From mice it frequently moves to rats, and it has been found on them in various parts of Europe, America, Australia, and Japan. It occasionally bites man, but evinces little inclination to do so.