SummarySystemic interactions of long duration among the Group I] Citrus Viroids, CVd‐IIa and CVd‐IIb were investigated by grafting buds from established citron (Citrus medicaL.) sources containing single viroids to a common healthy seedling. In healthy tissues, the two viroids became established in approximately equal titres as a mixed infection. By contrast, tissues that grew from the CVd‐IIa or CVd‐IIb source buds contained only trace amounts of the heterologous invading viroids.This interference between CVd‐IIa, a mild exocortis agent, and CVd‐IIb, the cachexia agent, was also demonstrated in the presence of CEVd, the severe exocortis agent but not a Group II viroid. When a CVd‐IIb bud was propagated on a citron containing CVd‐IIa, a dramatic reduction in the titre of CVd‐IIb was observed.The interference between CVd‐IIa and CVd‐IIb indicates that the mild and relatively innocuous isolate, CVd‐IIa, can interfere with the replication and/or accumulation of the severe cachexia agent, CVd‐IIb, in citrus. This offers a potential practical approach for the control of cachexia in commercial plantings