Tensaw virus, a new member of the Bunyamwera arbovirus group (named for the river bordering the area in south Alabama where the prototype strain was found), is sensitive to sodium deoxycholate, relatively heat-stable at 37°C, and filterable through membranes with an APD of 220 millimicrons, but not through 100 millimicrons. The virus produced clinical disease and death in suckling and adult mice but did not produce disease in rabbits, guinea pigs, or hamsters. The virus grows well in hamster-kidney, HeLa, duck-embryo fibroblast, and monkey-kidney tissue-culture systems, producing cytopathic effects and plaques. Detailed antigenic comparison of eight strains of Tensaw virus with five known Bunyamwera-group viruses was made and the relation of the Tensaw strains to other Bunyamwera viruses was discussed.