We need a fuller understanding of the feeding habits of fleas on rodents, both to help in the study of the epidemiology of plague and also in carrying out laboratory experiments on the same subject. In an earlier paper (Buxton, 1938) I described a ‘synthetic mouse hole’ in which I kept a rodent with a population of fleas (Xenopsylla cheopis) under controlled conditions of climate and feeding for periods of many days; with this I obtained some measure of the progeny which resulted under a variety of conditions. There was evidence that the mouse ate a number of the fleas, and I never controlled this variable, which was erratic and sometimes very large. We attempted to avoid the destruction of the fleas by feeding them on a baby mouse, which was changed at intervals, so that there was always one in the experimental vessel. In a postscript to the above paper it was shown that the baby mouse does not consume the fleas, almost all of which are alive when the experiment has lasted a week; in that period the adult mouse may eat about half or more of the fleas. Further experiments are in progress on the factors which determine the destruction of fleas by the mouse, and on the proportion of fleas which are actually found on the mouse under different conditions of temperature and so forth. In the present note, I shall confine myself to studying the baby mouse, as a host of X. cheopis. All the experiments here described were carried out between 1937 and 1939, though it has not till now proved possible to tabulate and consider the results. The execution of the work was in the hands of Mr S. Smith, whose help is acknowledged.