Allied species of the subgenus Leptotrombidium are often very difficult to classify. The author attempts to describe a new species belonging to this subgenus with special emphasis on the morphological stability and the distributional character covering a northern broad area of Honshu, Japan. Leptotrombidium (Leptotrombidium) asanumai n. sp. Body reddish orange, the general appearance of typical Leptotrombidium. Gnathosoma : Palpal pilous formula N-N-BNN. Scutum : The posterior margin is gently curved. AM seta and PL setae (longer than AL setae, measuring 74 long in holotype) have minute and sparse spikes. Sensillae are placed slightly posterior to the level of PL setae, and the proximal half of them is naked. The scutal measurements are almost same in specimens from four different localities. Dorsal setae : Pilous formula is 2-8-6(7)-6-4(6)-4-2(32-35) with little variation. They have the similar spikes as PL setae. Humeral setae measure 69μ long in holotype. Ventral setae : Comprise about 20 anteroanal setae and about 10 posteroanal setae with the similar spikes as DS setae. Coxal setae : One seta is out of anterior margin of coxa III. Ecological data : Eighty-eight larvae ex Microtus montebelli, 1 larva ex Apodemus speciosus, 1 larva ex Rattus norvegicus in 7 localities of Aomori Pref., July 1967 to Nov. 1976; 457 larvae ex M. montebelli in Takanosu, Akita Pref., Aug. 1969. The peak of seasonal occurrence was found from July to November by the author (1976) in Hirosaki Park. Type specimens : Holotype and 7 paratypes preserved in the Department of Parasitology, Hirosaki University School of Medicine, Hirosaki, Japan. Remarks and specific diagnosis : This mite is identical with L. sp. No. 50 by Sasa (1956) and L. sp. by Ito and Obata (1961). Historically, Asanuma et al. first obtained it in Oze, Gunma Pref. in 1950,then, Obata et al. found it from a locality of upper valley of the river Shinano, Niigata Pref., but it remained unnamed until today. This mite differs definitely from two related species L. akamushi and L. scutellare, because the scutal size and situation of sensillae are dissimilar, PL setae are relatively longer, the bodies of AM, PL, DS and VSP setae are more stout (4.5-6μ wide vs. about 3.5μ wide in L. akamushi), the spikes are more minute than those of the latter, and also the number of DS setae are less. The author found no difference between the present materials and the specimens of L. sp. No. 50 taken from Oze which was kindly supplied by Dr. K. Asanuma. The species is dedicated to Dr. Kiyoshi Asanuma, The National Science Museum, Tokyo, for the memory of his continual contribution to medical acarology.