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The ecology of a mosquito borne virus involves a consideration of the growth of the virus within the vertebrate and arthropod hosts, the reaction of these hosts to the presence of the virus, and the vertebrate-arthropod associations which are necessary for the dissemination and maintenance of the virus. Little is known of the physiological properties of a particular host and virus which permit multiplication of the virus. In this review stress will be laid u pon studies which attempt to discover the mechanisms in nature which maintain these viruses. The first section will relate to the geographical distribution and time of occurrence of the viruses as indicated by clinical disease in human beings and domestic animals, antibody surveys among various animal species, and attempts at isolation of viruses from arthropods and other animals. Section two deals with transmission and viremia studies with suspected vectors and vertebrate hosts and virus cycles in the laboratory. Section three is concerned with field observations attempting to demonstrate that suspected vectors and their vertebrate hosts occur in sufficient numbers and are so associated that a virus cycle is possible. Section four deals with the problem of maintenance of virus during prolonged cold or dry periods. Section five pertains to serological relationships of interest from the standpoint of virus evolution. Some of the newly described viruses are described in section six.