CLARKE1 showed that a strain of tick-borne encephalitis virus, ‘A52’, isolated in Finland2, is antigenically identical with the Central European tick-borne encephalitis viruses. Sokol et al.3 reported that the particles of the ‘Hypr’ strain of Central European tick-borne encephalitis virus purified from mouse brain suspension were predominantly spherical, and about 30 mµ in size. The same strain in HeLa cells has been demonstrated to be round-shaped with a dense inner body and a transparent outer zone surrounded by a surface membrane4. In the present work the negative staining technique5 was used to determine the size and shape of particles of the strain ‘A52’, grown in tissue culture.