Silverleaf whitefly, Bemisia argentifolii Bellows & Perring, adults and nymphs feed on the phloem tissue of host plant leaves. Infested leaves were rapidly fixed to prevent the insects from withdrawing their mouthparts. The adult stylet bundle enters the labial groove of the labium between the first and second segments and is completely contained within the labium except during feeding. Stylet length is equal to the combined length of labial segments 2, 3, and 4. Stylet penetration can be determined by examining the position that the stylet enters the labial groove. The physical force necessary for adult stylet penetration is derived from changes in the position of the whitefly head during feeding. The head is bent over the labium, which is attached to the leaf surface, forcing the stylet bundle down the labial groove and into the host tissue. Nymphal stylet lengths were found to be longer than previously reported, and of sufficient length to reach the phloem tissue in cotton and hibiscus leaves from essentially any position on the abaxial leaf surface.