An undescribed, maternally heritable, rod-shaped bacterium (or ''tertiary symbiont'') was detected by microscopy in hemolymph of about half (59/122) of pea aphid [Acyrthosiphon pisum (Harris)] clones collected from widely separated locations in California, On the basis of molecular phylogenetic analysis of 16S rDNA sequences, the bacterium was clearly placed among other Rickettsia in the a-subgroup of Proteobacteria, close to Rickettsia bellii - a rickettsia found in ticks. A PCR assay was developed to detect this bacterium in pea aphid clones with specific 16S rDNA PCR primers. Results of PCR-based assays completely correlated with detection by microscopy. This is the first confirmed detection of a Rickettsia in a herbivorous insect.