At the commencement of June, 1911, whilst engaged on plague epidemiological observations in the United Provinces my attention was drawn to the fact that nearly all the young Mus rattus that were being trapped and examined by our staff in Lucknow were females. It was this strange phenomenon, the parallel of which I had never encountered during a five years’ experience of plague research in India, that prompted the inquiry, the results of which are briefly set forth in this paper. A few words explanatory of the methods employed in the daily routine examination of rats will show the nature of the material at my disposal. The prime object in trapping and examining large numbers of rats was, of course, to ascertain how far facts thus collected would assist us in solving the plague epidemiological problems with which we were faced. The species, sex, and weight in grammes of each rat caught were noted; the address of the house in which the rat was trapped and the number and species of fleas found on it were recorded. The sex of each rat was noted after dissection of the animal, and if it were female a further note was made as to the existence of pregnancy. If pregnant the number of fœtuses was likewise written down. Finally, any pathological or other condition calling for comment was fully described.