1. Microscopical examination of the kidneys of 850 Rattus norvegicus killed in various parts of England and Wales showed an infection rate of 43% with leptospires, assumed to be Leptospira icterohaemorrhagiae for reasons stated.

2. Infection rates calculated from the whole series were the same for rural, urban and sewer rats, but there were some local differences as between one area and another for both surface and sewer rats.

3. An unusual feature was the finding of a 35% infection rate among young rats.

4. In another series of 219 rats taken alive, agglutination tests and kidney cultures gave an infection rate of 43%. Kidney sections of 69 of these rats were also examined microscopically and the results of the different methods of examination were analysed.

5. A similar survey was made on R. rattus. Only 2 of 88 caught in London were positive, compared with 31 of 35 killed in other towns. An additional 98 R. rattus killed during the fumigation of ocean-going steamers were examined, and 69 were found to be infected.

I should like to record my thanks to the following members of staff at the Infestation Control Division of the Ministry of Agriculture, Fisheries and Food, Tolworth, Surbiton, Surrey, for their help in this survey: Mr C. J. Armour, Dr E. W. Bentley, Dr W. P. Crowcroft, Mr R. A. Davis, Miss Y. Larthe and Miss M. Lawrence. Also to the following Field Officers of the Ministry who supplied material for this investigation: Mr A. F. R. Brown, Mr R. J. Clark, Mr G. R. Hill, Mr A. C. Hopkins, Mr R. S. Jailler, Miss B. B. Jones, Miss M. Neve, Mrs J. C. Russell and Mr S. R. Surtees.

I wish also to acknowledge the valuable assistance received from members of my technical staff, especially Miss Joan Messum and Mr R. J. Reed.