The molecular size limitations of the digestive system, including the filter chamber of immature and adult Bemisia tabaci (Gennadius) B biotype (=B. argentifolii), were studied by tracking the movement of fluorescent-labeled molecules and microspheres ingested by whiteflies. Soluble fluorescent molecules and labeled dextrans, ranging from 389 to 2,000,000 Da, were observed throughout the digestive tract of immatures 10–30 min after feeding was initiated. After removal of labeled molecules from the diet, fluorochromes were cleared from digestive system of immatures within 2 h. Fluorescent-labeled 0.1- and 0.2-μm microspheres were ingested by larvae and saturated the digestive system within 2 h after initiation of feeding. Large, 0.5-μm spheres were not observed in the digestive tract of immatures, probably because singly or as aggregates, they were too large to enter the stylet food canal. The smallest spheres examined, 0.02 μm, were not detectable in the digestive tract of immatures. Observations for whitefly adults were identical to those for larvae, with two exceptions. In adults, soluble fluorochromes were detectable1 h after feeding commenced, and 0.02-μm spheres were observed primarily in the esophagus, filter chamber, anterior midgut, and hindgut, but not in the posterior portions of the midgut. We hypothesize that most of the 0.02-μm spheres ingested by adult whiteflies were shunted directly to the hindgut by way of the filter chamber, effectively bypassing the midgut. This is, therefore, a feasible route for virions of the plant pathogenic genus Begomovirus, which are of similar size to the small microspheres and are transmitted in a circulative manner by B. tabaci.