Cloned DNA-B components, belonging to the bipartite begomoviruses Indian cassava mosaic virus (ICMV) and Sri Lankan cassava mosaic virus (SLCMV), family Geminiviridae, when co-inoculated along with previously cloned DNA-A components of the respective viruses onto the experimental host Nicotiana benthamiana, generated defective DNAs (def-DNA) ranging in size from 549 to 1555 nucleotides. All the cloned def-DNAs contained the common region (CR) as well as portions of either DNA-A or DNA-B and, in a few cases, both DNA-A and DNA-B, representing recombinant products, the junction points of which correspond to repeats of 2-11 bases found in the parental molecules. The DNA-B-derived def-DNAs were, in some cases, associated with a decrease in levels of DNA-B, with a concomitant change in the symptoms from downward leaf curling in the older leaves to upward leaf-rolling in newly emerging leaves, more typical of monopartite begomoviruses.