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Examination was made of 13,000 individuals of Clethrionomys plareolus (S c h r e b e r, 1780) and 4,000 individuals of Clethrionomys rutillus (Pallas, 1779) obtained in the taiga forests covering the region between the rivers Wiatka and Kama, over a period of 5 years, most often from May to August. Several groups were distinguished on the basis of differences in length of root and height of crown of M 2. After comparing the times of beginning and end of reproduction, periods of appearance of the maximum number of litters in a population with a defined type of tooth structure, it proved possible to establish the absolute age of each of the groups distinguished. A key is given to identification of the absolute age of the two species of vole examined. Up to the second month of life, the teeth consist of prisma only, the neck of the tooth not being formed until the voles are two months old. At the age of 3-4 months the roots form less than 1 /* of the whole height of the tooth. At the age 5-6 months this relation is Vi, 7-8 months-9-10 months-Vs and at the age of 11-12 months-2 h. In the case of animals over one years old only the roots of the tooth remain, or part of the neck may still be seen.

Three identical strains of an arbovirus were isolated from 475 Ornithodoros papillipes ticks collected in June, 1972, in burrows of the great gerbil (Rhombomys opimus Licht., 1882) in the environs of Beshkent, Karshinsk steppe, Uzbek S.S.R. The isolate was found to range among flaviviruses. Complement-fixation, agar diffusion precipitation and neutralization tests is tissue culture and mice indicated a one-way antigenic relationship between the isolate and West Nile virus. However, the pattern of differences between them made it possible to consider the isolated agent as a new virus, "Karshi" virus. The results of electron microscopic studies of this virus are presented.

Eleven virus strains were isolated from ticks Hyalomma asiaticum asiaticum Schulce et Schlottke, 1929, and Hyalomma plumbeum plumbeum Panzer, 1796,collected in 1971-1974 in desert regions of the Uzbee S.S.R. and the Turkmen S.S.R. According to CF test the strains were closely related to each other and not antigenically connected with viruses from antigenic groups A, B, California, CHF-Congo, Bakau, Bunyamwera, Gajam, Kaisodi, Qalyub, Kemerovo, Quaranfil, Simbu, Turlock, Hughes, Uukuniemi, Tete and 21 ungrouped viruses isolated from ticks. The virus was named "Tamdy" after the place of isolation of a prototype strain LEIV-1308 Uz. The virus does not agglutinate goose erythrocytes, it is pathogenic for suckling mice and 3 weeks old mice by intracerebral infection. Replication of a virus with CPE in cell cultures--L, Rh, A1--and without CPE--in pig embryo kidney cell cultures--was demonstrated. According to ultrafiltration and electron microscope data the size of the virus is about 90 nm. It is rather sensitive to lipid solvents and is an RNA-virus. Morphologically the virus resembles the Bunyaviridae.

SummaryIn 1969--1971 nine virus strains were isolated from Ixodes putus ticks and were shown to be antigenically related to Kemerovo virus by complement-fixation tests. By neutralization tests, no antigenic relations were revealed. Five virus strains were isolated on Tyuleniy island, Sakhalinsk region (1969--1971); three virus strains on Ariy Kamen' rock in the vicinity of the Pacific coast of Bering island, Commodore islands, Kamchatsk region (1970--1971) and one on the SouthEastern coast of Chukotka, Magadansk region (1971). The isolated virus was named after the place of its first isolation, the sea of Okhotsk. A serological survey of 1066 birds in the Far East indicated their active role in circulation of the virus, since complement-fixing antibodies were found in 3.9 per cent of Uria aalge (common murres), 14.6 per cent of Fulmarus glacialis (fulmars) and 0.7 per cent of Phalacrocorax pelagicus (pelagic cormorants).Complement-fixing and virus-neutralizing antibodies were found in birds from Commodore islands in Kamchatsk region and from Iona, Tyuleniy, Moneron islands in Sakhalinsk region. IntroduetionTicks were collected in 1969--1971 on: i) Tyuleniy island, Zaliv Terpeniya Patience bay) of the sea of Okhotsk, S~khalinsk region (12) ; ii) Ariy Kamen' rock situated close to the Pacific coast of Bering island, Commodore islands, Kamchatsk region (16); iii) the South-Eastern coast of Chukotka, Magadansk region.