Student Number : 0314429G - MSc research report - School of Molecular and Cell Biology - Faculty of Science / Tomato (Lycopersicon esculentum) is a horticultural commodity of great economic importance in many parts of the world, including South Africa. A previous study identified a new begomovirus, Tomato curly stunt virus (ToCSV), as the causative virus of a new and potentially devastating disease of tomatoes in South Africa. In this study, symptomatic plants, suspected of infection with an uncharacterized ToCSV isolate (01/2521) were collected for screening from Pietermaritzburg, South Africa. A host range study was conducted with the original ToCSV isolate (99/0631). Two small DNA molecules (1449 nts and 755 nts) were found associated with ToCSV [01/2521] using near-full length primers AL1c2745 and PAR1v32 specific for ToCSV. A single small DNA molecule (842 nts) was also found in association with the original ToCSV isolate. Nucleotide sequence analysis revealed that the two small DNA molecules (1449bp and 755bp) have no significant nucleotide sequence identity (less than 20%) with any known begomovirus. The 842bp molecule has the most significant nucleotide sequence identity (48%) to that of ToCSV (AF261885), while less than 20% nucleotide sequence identities were found when compared with other begomoviruses. Nucleotide sequence alignment of the 842bp DNA molecule to the ToCSV sequence, showed that this small DNA molecule is a chimeric molecule that could have arisen through recombination, partly from the coding regions of the ToCSV genome, but the rest of the molecule is of unknown origin. All three small DNA molecules identified in this study were compared to some known begomovirus associated subgenomic molecules and satellite molecules, and sequence identities of less than 20% were found. To our knowledge, this is the first report of a small DNA molecule found associated with the ToCSV genome. The complete genome sequence of ToCSV [01/2521] was not determined. Based on the results we obtained from the host range study, all the chosen test plants are not susceptible to ToCSV infection. The infectivity of all the small molecules identified in this study, is currently being investigated.