SUMMARY To increase our understanding of the mechanisms of insect-transmission of begomoviruses we studied the distribution of coat protein (CP) and DNA of Tomato yellow leaf curl Sardinia virus (TYLCSV) in its insect vector (Bemisia tabaci), and in tomato plants (Lycopersi- con esculentum). The study was extended to other bego- moviruses and a potential overwintering host plant for TYLCSV, black nightshade (Solanum nigrum). Im- munogold labelling (IGL) showed that a polyclonal an- tiserum against the coat protein of African cassava mosa- ic virus (ACMV) (ACMV-CP-As) cross-absorbed with healthy plant and insect tissue, cross-reacted specifically with both homologous (ACMV, non-transmissible iso- late), and heterologous ((TYLCSV and Asystasia golden mosaic virus, (AGMV)) viruses. ACMV-CP-As revealed that TYLCSV is located in mature and immature sieve elements, the nuclei of the companion cells, and the cy- toplasm of the phloem parenchyma cells of L. esculen- tum and S. nigrum, with a similar pattern of distribution in both species. In situ DNA-hybridization (ISH) using a specific TYLCSV-DNA-probe showed that the TYLCSV-DNA occurred mainly in the vascular tissues of stems and roots of S. nigrum. In some leaves the in- fection spread to the palisade parenchyma, confirming that the DNA of the virus is not phloem-restricted in this host. IGL of viruliferous B. tabaci detected both TYLCSV and AGMV in cells of the filter chamber, whereas the non-transmissible isolate of ACMV was detected only in the lumen of the insect gut. No labelling was obtained in non-viruliferous individuals of B. tabaci. The anti- serum labelled, for the first time, the primary glands of the salivary gland system of TYLCSV-viruliferous B. tabaci, suggesting that the circulative pathway of TYLCSV can be completed as intact virions.