Whiteflies (family Aleyrodidae) possess heritable eubacterial endosymbionts sustained in specialized organ-like structures called mycetomes. Comparisons of distances between the ash whitefly, Siphoninus phillyreae, and two biotypes (''A'' and ''B'') of the sweetpotato whitefly, Bemisia tabaci, based on sequence analysis of genes for 18S rRNAs (rDNAs), were equivalent to the distances represented by the 16S rDNAs of their respective endosymbionts. This finding indicates that evolutionary divergence in whitefly hosts and their endosymbionts is congruent. The nucleotide sequences of the 18S rDNAs and endosymbiont 16S rDNAs indicate the two biotypes of B. tabaci are the same species.