A bacilliform virus, designated orchid fleck virus (OFV), was isolated from six genera of orchid plants including Angulorea, Cymbidium, Dendrobium, Odontoglossum, Oncidium, and Pescatorea. The virus causes systemic leaf symptoms of chlorotic or necrotic flecks in orchid plants. The virus was transmitted to Dendrobium, tobacco, Nicotiana glutinosa, Chenopodium amaranticolor, and three other plants, by sap inoculation when the temperature was higher than 30C. The virus particles could be detected in preparations obtained by the modified direct negative staining method devised by the present authors. The virus was partially purified, and the preparation contained numerous virus particles and was infectious to Dendrobium plants. The virus particle is approximately 40×150nm in negatively stained preparations, but 32-35×100-140nm in thin sections. The particle had no envelope and consisted of a helical structure with a pitch of 4.5nm. In thin sections of OFV-infected tissues, a large number of virus particles were found in the nuclei and in the cytoplasm, and the viroplasm development was also observed in the nuclei of OFV-infected cells. A series of electron micrographs suggest that OFV particles are formed in the viroplasms and disperse along the membrane system into the cytoplasm after passing through the nuclear envelope. From these observations it is concluded that OFV is a virus belonging to a new virus group different from the rhabdovirus group. Dendrobium virus (Petzold, 1971), Phalaenopsis virus (Lesemann and Begtrup, 1971), ringspot virus of coffee (Kitajima and Costa, 1972) citrus leprosis virus (Kitajima and Costa, 1972) may be identical with, or closely related to, OFV.