A spirochaete, S. graingeri n.sp., was isolated from batches of Ornithodoros graingeri from caves near Tiwi, south of Mombasa. Very scanty infections were caused in white rats and mice, and a persistent parasitaemia in a general paralytic. Guinea-pigs, a monkey, and a young rabbit were insusceptible. There was evidence that unlike most tick-borne strains the spirochaete was only slightly neurotropic.