Fleas are of great importance as vectors of disease in many parts of the world. Public health workers are most concerned with fleas that carry the organisms of bubonic plague and murine typhus from rats to man and those that transmit plague among wild rodents and occasionally to man. Many people are concerned with fleas that attack domestic animals, serve as intermediate hosts for some species of dog and rodent tapeworms, as vectors of Salmonella bacteria, the bacteria causing tularemia. Public health workers should be familiar with the species that attack man, domestic animals, rats, wild mammals, their habits and life histories, in order to apply the most effective control methods, and the most effective insecticides to control these insects.