The present paper reports that in northern Texas an acariasis of the human skin is caused by bites of a blood-sucking rat mite (Leiognathus) Liponyssus bacoti Hirst. Attention is invited to the history of the parasite, to the types of lesions caused by bites of the mites, and to attempts to correlate the presence of these mites with the occurrence of endemic typhus fever. HISTORICAL The tropical rate mite was first recorded from rats in Egypt. In 1913, specimens of the mite collected at Assuit, Egypt, were described by Hirst1 as Leiognathus bacoti. A few months later other specimens were obtained from Egypt, Abyssinia, Australia and Argentina. The latter specimens were recognized by Hirst2 as the same species. For them he considers the brown rat, Mus norvegicus, as the principal host. The tropical rat mite was first reported2 as a blood sucker of man at Sydney, Australia.