To the well-known list of rat poisons, such as arsenic trioxide, red squill and zinc phosphide, the last years of the War added another, a-naphtha-thiourea. Dr. C. P. Richter, of Baltimore, was responsible for showing that its toxicity to Rattus norvegicus was high (A.L.D./50, 10 mgm. per kilo body-weight). Somewhat extravagant claims were made for its effectiveness when applied as a direct bait. In none of the field tests were accurate censuses made of the rat population before treatment. Prolonged arid detailed experience in Great Britain at the Bureau of Animal Population has shown that knowledge obtained by bait census is essential before field trials are undertaken. The use of a-naphtha-thiourea as a dusting powder, however, opened up possibilities of a new method of application, but its usefulness depended upon the safety factor involved. Toxicity tests made on other animals showed an enormous range of sensitivity. R. rattus, the ship or black rat, had an A.L.D./50 of 250+ mgm. per kilo body-weight, more than twenty-five times that for R. norvegicus, while Mus musculus, the house mouse, had an A.L.D./50 of 80-90 mgm. per kilo body-weight. These data showed that it was impossible to predict a safety-level from one species to another. Consequently, although tests on monkeys (Macacus sp.) showed a high resistance, it was dangerous to assume that man was equally immune. Dusting application was therefore not advised in Great Britain Until more information was available.