THE occurrence of disease is a function of population characteristics. Its spread depends upon movements, its transmission upon contacts, and even its presence upon density of population. Hence, it is hardly surprising that the control of disease requires a knowledge of characteristics of the population involved. In particular, the control of ratborne diseases requires a knowledge of rat populations. Rat populations throughout the world arie relatively similar, although local conditions and specific differences produce some variation in degree. This paper, therefore, discusses the general characteristics of global rat populations, although most of the generalizations are based upon studies of the Norway Rat (Rattus norvegicus) in Baltimore. First, let us consider the simple case of the growth of a population in a stationary environment. Let us assume that a number of rats are introduced into an area that has ample and constant supply of food and harborage. The rat population will increase till it reaches a limit determined by the food and harborate