Bacterial 16S rDNA amplified by PCR from the pea aphid Acyrthosiphon pisum included a sequence with >98% similarity to secondary symbionts in the whitefly Bemisia tabaci. The ‘pea aphid Bemisia-like bacterium’ (PABS) and B. tabaci secondary symbionts are estimated to have diverged 17–34 million years ago, a time considerably more recent than the common ancestor of aphids and whitefly and suggestive of horizontal transmission of this bacterial lineage. PABS was scored in both the gut and ovaries of aphids by PCR and identified as a small rod by in situ hybridisation. PABS was not universal in pea aphids: 2/3 laboratory strains and 13/35 of field aphids were PABS-positive. It is suggested that the incidence of PABS in pea aphids is determined by the balance between loss (processes may include occasional failure of vertical transmission and selection against PABS-positive aphids) and horizontal transfer between insects.