The relative infestation of small mammals by fleas was studied in northwestern Illinois. Eleven host species numbering 718 individuals were collected from February to October in 1958. Three hundred eighty-six fleas of 11 species were taken from these hosts. More fleas per host were collected in the cooler months than in the warmer months. Orchopeas leucopus, a flea often considered to infest Peromyscus without exhibiting an apparent preference as to species, may actually be more common on species which occupy woodland habitats. Mean numbers of fleas of each of three species on Peromyscus leucopus, a host species taken in all seasons, indicated that peak populations of the three species do not coincide. A relatively large number of shrews and associated small mammals were trapped during a survey for rabies in shrews between February 14 and October 3, 1958. Fleas were removed from these specimens and submitted for identification to the Section of Faunistic Surveys and Insect Identification, Illinois Natural History Survey. Because relatively little is known about fleas in this section of the Midwest these records are believed to represent a contribution. Acknowledgment.-These observations were supplementary to an investigation of sylvan rabies which was supported by a grant (No. E-1349) from the Division of Research Grants, U. S. Public Health Service, Department of Health, Education, and Welfare. Thanks are due Dr. C. 0. Mohr for suggesting the study and for a critical reading of the manuscript and to Dr. L. J. Stannard for identifying the fleas. Also, Drs. T. G. Scott and R. E. Yeatter, and Mr. G. C. Sanderson gave help and advice which is gratefully acknowledged. METHODS Ninety-five trap lines, each consisting of 20 stations 10 yards apart with two break-back traps per station, were set along secondary road rights-of-way in Jo Daviess, Carroll, and Whiteside counties. The traps, baited with liver sausage to attract shrews, were allowed to remain in place for two consecutive nights. The traps were examined each morning, captured animals removed, and the bait replaced when necessary. Specimens obtained were placed in individual No. 1 paper bags. In the laboratory, the contents of each bag were dumped into the center of a piece of brown wrapping paper. The sack was torn apart, and fleas clinging to the bag or to debris were collected with forceps and placed in a vial of 70 per cent alcohol. The mammal carcass was then brushed and combed thoroughly with the points of the forceps, and additional fleas clinging to the host were collected. Mammals which were badly deteriorated or partially eaten by other animals were not examined for fleas.