The whitefly, Bemisia tabaci Genn., is known worldwide as either the cotton, sweet potato, or the tobacco whitefly. Although recognized since the turn of the century as a phytophagous pest, and shortly thereafter as an important vector of plant viruses, the insect has only recently become of international concern as the transport of plant materials and exportation of fiber and vegetable products has become increasingly more global in nature. Important changes in agricultural practices, and the need to meet world food demands have likely precipitated the change in status of B. tabaci from a member of the endemic fauna to that of a major deterrent to agricultural productivity. The taxonomic synonymization of at least eighteen designated worldwide, species of Bemisia into the single genus and species, B. tabaci, translates into the realization that this polyphagous insect is worldwide in distribution throughout the tropical/subtropical latitudes with a recognized host range comprised of greater than 500 plant species. In earlier reports from tropical regions, host-related affinities or preferences among local populations, termed "races," or "biotypes" of B. tabaci were described. Experimental evidence for the concept was derived from studies designed to ascertain biological host ranges of regionally important plant viruses. In the context of these studies, it was not always possible to determine virus host ranges because laboratory colonies of B. tabaci could not survive on certain test plant species long enough to transmit the virus in question. Thus, successful feeding and subsequent reproduction were recognized as essential to the compatibility of the whitefly-host interaction, and, in most cases to the transmission of the virus. At times, however, continuous exposure to a plant species by a B. tabaci population with strong host preferences resulted in the ability to colonize additional hosts. Thus, the terms race, or biotype were used to refer to populations with recognized host preferences. These data were considered inferential due in part to the lack of corroborating information on genetic polymorphism, or distinguishing morphological characters. The surgence of B. tabaci as a major pest and virus vector during the past decade has stimulated investigations into the biology, behavior, and insect host interactions on a worldwide basis. Recent studies in Africa, Columbia, Israel, and in the United States have prompted a renewed interest in the concept of biotypes of B. tabaci. The coincidental emergence of the so-called "new poinsettia strain or 'B' biotype" in the US, and the resulting impact on agricultural and horticultural productivity have reaffirmed the need to understand more about fundamental differences among populations of B. tabaci.