1. I was engaged in a survey of rodents and their parasitic fleas in Inner Mongolia from February 1944 to June 1945. I would like to discuss in this paper information obtained on house-rats and their fleas. 2. I collected 147 Rattus norvegicus norvegicus and 15 Mus molossinus in 6 districts of Inner Mongolia. 3. The flea fauna of Rattus norvegicus norvegicus were comprised of 4 genera and 4 species as follows : a. Xenopsylla cheopis (Rothschild, 1903) -289 (65%) b. Monopsyllus anisus (Rothschild, 1908) -128 (29%) c. Nosopsyllus fasciatus (Bosc, 1801) -26 (6%) d. Pulex irritans Linnaeus, 1758 -5 (1%) 4. The average number of fleas per Rattus norvegicus norvegicus was 3.0 and the average number of Xenopsylla cheopis per R. n. n. was 2.0. 5. The house-rat and flea fauna of Inner Mongolia is similar to that of North China.
