The occurrence of Rocky Mountain spotted fever due to infection with Rickettsia rickettsi in the eastern part of Long Island, N. Y., is not infrequent, because of the prevalence of the common vector Dermacentor variabilis, the dog tick. There are usually 10 to 15 cases each year in Suffolk County. The mortality of the disease is known to vary from year to year and in different parts of the country from 15 per cent to as high as 37.5 per cent in the Bitter Root Valley. The following case is reported because of the striking and rapid result obtained with aureomycin, a new antibiotic which has been found to be extremely effective both in vitro and in vivo against rickettsial organisms of several varieties, as well as against certain viruses and bacteria. Aureomycin has been produced from a strain of Streptomyces aureofaciens isolated by Dr. B. M. Duggar of the