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To elucidate the co‐evolutionary relationships between phloem‐feeding insects and their secondary, or facultative, bacterial symbionts, we explore the distributions of three such microbes — provisionally named the R‐type (or PASS, or S‐sym), T‐type (or PABS), and U‐type — across a number of aphid and psyllid hosts through the use of diagnostic molecular screening techniques and DNA sequencing. Although typically maternally transmitted, phylogenetic and pairwise divergence analyses reveal that these bacteria have been independently acquired by a variety of unrelated insect hosts, indicating that horizontal transfer has helped to shape their distributions. Based on the high genetic similarity between symbionts in different hosts, we argue that transfer events have occurred recently on an evolutionary timescale. In several instances, however, closely related symbionts associate with related hosts, suggesting …