A short description of the tsutsugamushi disease is given, followed by a note on the conditions under which it occurs in Sumatra and the Malay States. Abandoned agricultural land which has grown up in weeds is particularly dangerous. Small rodents are the reservoirs of the virus which is carried from them to man by larval mites. Trombicula akamushi is the carrier in Japan, and T. deliensis in Sumatra. The disease has been conveyed to monkeys by inoculation and also by the bites of infected mites. A description is given of four mites which are commonly found in the ears of rats in the Malay States, and a method for the examination of the mites is described. Black rats, to the number of 130, were examined. Sixteen were trapped in an abandoned portion of an oil-palm estate where three Europeans became infected with the tsutsugamushi disease, and T. deliensis, the Sumatran carrier, was found on ten of them. Thirty rats were caught in other parts of the estate, and T. deliensis was found on nine. Thirty rats were caught in other rural districts, where the disease was not known, and T. deliensis was found on only one. Fifty-four rats caught in the town of Kuala Lumpur were examined, with the result that T. deliensis was found in none. The conclusion is reached that T. deliensis is probably the carrier in the Malay States, as it is in Sumatra.