The whitefly Bemisia tabaci (Gennadius) is one of the most threatening pests to crop production in tropical and subtropical regions. Severe damage results from its effective transmission of plant viruses; the most serious are members of the genus Begomovirus. B. tabaci is a complex of at least 34, species with Middle East-Asia Minor 1 (MEAM1) and Mediterranean being the most aggressive due to their broad host range and enhanced resistance to pesticides. Because invasion of these species into newareas is often linkedwith severe outbreaks of new virus diseases, a comprehensive knowledge of the abundant whitefly species is a prerequisite for efficient and sustainable crop management. Species distribution of this important pest and plant virus vector is barely known, and therefore a study was undertaken to elucidate distribution and prevalence of B. tabaci in major crops of Thailand and Vietnam. In Thailand the indigenous B. tabaci Asia1 was the dominant species throughout the regions and none of the invasive species was present. In contrast, a high abundance of MEAM1 was observed in Vietnam. From its distribution in theNorth of the country an invasion fromSouthern China and replacement of indigenouswhitefly species could be delineated. Laboratory experiments showed that MEAM1 was able to replace the Asia II 1 species indigenous to this region within four generations. In virus transmission studies MEAM1 also showed higher rates of Tomato yellow leaf curl virus transmission compared to the Tomato leaf curl Hainan virus also indigenous to Vietnam.