SUMMARY1The occurrence in East Africa is recorded of a virus producing a transitory diffuse mottling in maize. The symptoms are distinguishable from those of other viruses known to affect this plant.2The mottle virus is transmitted by active races of the same species ofCicadulinathat transmit the streak virus. Insects of an inactive race usually fail to transmit, although rare exceptions have been encountered.3The presence of one virus in a plant does not prevent the development of the other, nor are the ultimate symptoms of one affected by the other. In some circumstances, however, the mottle virus may cause a significant delay in the development of the symptoms of streak disease. The presence of one virus in an insect does not prevent it from taking up and transmitting the other.4The mottle virus is regarded as a new one, unrelated to that of streak disease.