Gene VI of the cauliflower mosaic virus (CaMV) genome encodes a protein (P(66)) in virus-infected plants that accumulates in cytoplasmic inclusion bodies. When a segment of the CaMV genome bearing gene VI is transferred to tobacco plants by the Agrobacterium tumefaciens Ti plasmid, the resulting transgenic plants display viral-like symptoms. Symptoms produced by the DNA from two different viral isolates (CaMV Cabb B-JI and CM1841) were distinct-symptoms from the first were mosaic-like, whereas the other caused uniform bleaching of leaves. That gene VI was responsible for the symptomatic phenotype was demonstrated by showing that symptom production was blocked by deletions and by a frame-shifting linker mutation in gene VI. Furthermore, in primary transformants, there was a strict correlation between the appearance of symptoms and the presence of gene VI product, P(66), detected by immunoblots. Hence, a protein encoded by the CaMV genome produces viral-like symptoms in transgenic tobacco plants.