During an investigation into dentofacial morphology and dental diseases in skeletal remains of the recent Ainu, several traits of tooth loss, probably due to ritual tooth ablation, were observed. Since KANESEKI (1940) predicted the existence of this ritual in Ainu, there was no study concerned with this problem in Ainu. In the present paper, it was aimed to announce that this ritual really existed in Ainu.Materials used in this investigation were 125 skulls of the recent Ainu, which were preserved in the University Museum, The University of Tokyo (Table 1). Traits of tooth ablation were observed in 19 cases, and the total number of extracted teeth was 45. Most of the teeth extracted were central and lateral incisors in both maxilla and mandible, and the ablation of the maxillary canines was observed in only two cases (Table 2 and Fig. 1). The frequency of the cases with ritual tooth extraction was 15.2% as a whole, and the highest incidence of 20.6% was observed in the Okhotsk coast area, except Sakhalin in which only one sample was obtained and it was the case of ritual ablation (Table 3 and Fig. 2).There were three cases out of thirty juveniles, 10.0%; 11 cases of 76 adults, 14.5%; two cases of ten matured people, 20.0%; and three cases of nine seniles, 33.3%. These facts may suggest that the extraction was not carried out at some decided ages such as adolescence, but it was done in more than one, or several occasions even after adulthood. There were 11 extracted cases out of 66 males, 16.7%; and six cases of 46 females, 13.0%; two cases of 13 unknown cases, 15.4% (Table 4).The differential diagnosis of ritual tooth ablation from the traits of tooth loss for other reasons contained very few difficulties. However, it seemed impossible to discriminate the ritual ablation from tooth loss due to accidental trauma. It was also difficult to confirm the ritual ablation in the case in which most of the teeth had been lost for old age (Fig. 3). In these points, the authors agree with HRDLICKA (1940) as well as with OHTAWA (1983), particularly concerning the shape of alveolar bone in the ablation cases that becomes kniferidge like, and the HRDLICKA'S expression, Such a diagnosis, with sufficient experience, is fortunately not overdifficult, seems really acceptable. Naturally, a question arose to the contrary opinion of MERBS (1968) that all the HRDLICKA'S 1165 cases out of 6645 samples from Siberia and America must be caused by unintentional trauma occasioned by use of the anterior dentition as a tool.Space closure after ritual tooth ablation was not dominant, but only slightly observed. Since KOGANEI (1919) pointed out that the extraction space closed itself with time, many authors made efforts to estimate the time of ablation, and the order of tooth extraction when more than one teeth were lost, and to find the real reason of this strange ritual. These discussions, however, do not seem to be fruitful, because the amount and the rate of space closure do not simply relate to the time factor, but they change depending on the extent of the toothto-denture-base discrepancy in each individual, or even in each jaw (INOUE, 1980; INOUE et al., 1983; Fig. 4). For this reason, there is a limitation in estimating the elapsed time after the tooth extraction only from the size of the remaining space.The type of ablation, in which the central and lateral incisors in both maxilla and mandible are extracted, is very close to those of the Siberians and the American natives (HRDLICKA), and differed from those in the Jomon people in Hokkaido (DoDo et al., 1984), and also from Japanese skeletal remains from the Jomon and Yayoi periods (Table 5). From these facts, it is presumed that there has been strong influences of the so-called Northern Cultures on the Ainu, since they had been isolated from the Japanese in Honshu, both culturally and physically, probably after the Jomon period.