Nearly all sternorrhynchan insects rely on bacterial endosymbionts to supplement a nutritionally poor diet of sap or plant material. Interactions between aphids and their endosymbionts have been an extraordinary model system for the study of co-evolution between symbionts and molecular evolution of endosymbiotic bacteria. The endosymbionts of psyllids, whiteflies and mealybugs have also been characterized and work is progressing on all these systems. However, the apparent diversity of endosymbionts in most scale insect families remains unexplored. For example, within the diaspidid scale insects it is unknown what type of bacteria they house as endosymbionts and how they are distributed within the family Diaspididae. Here we use 16S DNA sequence data to investigate the endosymbionts of diaspidids in a phylogenetic framework. We then compare these sequences to those of other endosymbionts of sternorrhynchan insects and discuss the implications for co-evolution between scale insects and their endosymbionts.