Virus content of symptoms expression have been investigated in CMMVB-infected wheat (var. « Etoile de Choisy ») at temperatures ranging from 10 to 29 °C. Necrotic symptoms were observed on the inoculated leaves only between 10 and 17 °C. Nevertheless only 6 h at 15 °C were required for the production of necrotic symptoms at 24 °C. Systemic infection of the plant by the virus could be observed only at temperatures exceeding 15 °C. Production of mosaic symptoms occurred only after transfers in temperature. They occured when the plants were kept at least 36 h at 24 °C before transfer to 15 °C. They appeared earlier and were more pronounced under intense illumination. Successive transfers between 24 °C and 15 °C resulted in a considerable increase in virus concentration that was in the order of 1 mg/g in the 8th wheat leaf. Conditions for production of necrotic and mosaic symptoms in tobacco mosaic virus-infected Nicotiana tabacum var. xanthi nc are basically different from those required in VMMDB-infected wheat. For the latter combination, virus necrosis and mosaic symptoms could be observed at the same temperatures. Once the systemic symptoms had appeared after sufficient time at high temperature, they remained after transfer to lower temperatures.