Mixed infections of two economically important viruses, Turnip mosaic virus (TuMV) in the family Potyviridae and Ribgrass mosaic virus (RMV) in the genus Tobamovirus, were studied ultrastructurally on oriental cabbage. TuMV-AC18 (alpine isolate in Korea) induced chlorotic spots on inoculated leaves of both ``SSD63`` inbred line known as susceptible to TuMV, and ``Tambok`` commercial cultivar, known as resistant to the virus, in the early stages of infection. TuMV-C5 (Taiwan isolate) caused severe mosaic and malformation on the upper leaves of ``SSD63``, and necrotic spots in both inoculated and upper leaves of ``Tambok``. RMV-CA1 (oriental cabbage isolate from alpine in Korea) induced vein chlorosis, leaf malformation, and midrib necrotic streak in the upper leaves of both ``SSD63`` and ``Tambok``. Both oriental cabbages infected with a combination of TuMV-AC18 and RMV-CA1 showed synergistic symptoms of severe yellowing, severe mosaic, and necrotic spot or vein necrosis on their leaves. A combination of TuMV-C5 and RMV-CA1 produced synergistic symptoms only in ``SSD63``. In ``Tambok`` infected with the combination of TuMV-C5 and RMV-CA1, the number of necrotic spots on the inoculated leaves was one half lesser than that on singly infected with TuMV-C5. A few necrotic spots progressed systemically. In cells infected with a combination of TuMV-AC18 and RMV-CA1 or TuMV-C5 and RMV-CA1, the particles of the two viruses made nonagon-like rings (NLR); one TuMV particle was surrounded loosely by nine RMV particles. Two unrelated viruses of TuMV and RMV were compacted in the central part of the spiral aggregates (SA) that was induced strikingly in cells by the mixed infections. The SA showed NLR in its center of the cross-sectioned side. Many particles of RMV of Tobamovirus were closely associated with Potyvirus-characteristic cylindrical inclusions. The SAs in the mixed infections were formed easily by the Potyvirus of TuMV-AC18 or -C5 isolates.