Electron- and immunoelectron-microscopic methods were used for detection and investigation of rabbit haemorrhagic disease virus (RHDV). The observed virus particles were uncoated, with an icosahedral symmetry, with a 35-40 nm dia and capsids built up by 32 capsomers with a central hollow part and a 3.5-5 nm dia. Particles with empty capsids could also be seen among them. Single virions visible in 5-fold symmetry, exhibited 10 projections on the surface. Virions with a 33-34 nm dia and cup-shaped surface depressions were a rare finding. Some of these particles were indistinguishable from the caliciviruses, but in most of them the cup-shaped depressions were single or no more then 2-3 in number, localized predominantly unilaterally or asymmetrically. Particles 25-29 nm large with full or empty capsids without capsomers could also be seen. Most probably they were some immature forms in the process of development of the RHDV.