The sweet potato whitefly Bemisia tabaci (Gennadius) is an exceedingly polyphagous pest capable of developing on hundreds of plant species. To date, more than 20 B. tabaci biotypes have been designated. Out of those, only the B and Q biotypes have been reported from Israel. B. tabaci harbors the primary symbiont Portiera aleyrodidarum, and a diverse array of secondary symbionts. This insect transmits the most devastating plant viruses which belong to Begomoviruses, Criniviruses and Ipomoviruses. Begomoviruses are one of the largest and most economically important plant virus groups. The best studied whitefly-virus interactions is the B. tabaci-Tomato yellow leaf curl virus (TYLCV) system. It has been shown that virus transmission is directly influenced by the symbiotic complement in B. tabaci. A symbiont-produced 60 kDa GroEL homologue binds to virions in the haemolymph, ensuring circulative transmission. To determine which of the bacteria is involved in the transmission of TYLCV, the GroEL genes of the symbionts Portiera, Hamiltonella and Rickettsia have been cloned in yeast and tested in the two-hybrid system for their ability to bind TYLCV coat protein (CP). It appears that GroEL from Hamiltonella of the B biotype, but not from Portiera and Rickettsia, is able to bind to TYLCV CP. The ability of the B. tabaci B and Q biotypes to transmit TYLCV and infect tomato plants was assessed. The possible involvement of the different B and Q symbiotic bacteria in the transmission efficacy will be presented.