Abstract. Sexually transmitted diseases (STDs) have rarely been reported in insects and other invertebrates. The majority of those reported involve organisms where sexual transmission is augmented by either vertical (i.e. inherited) transmission, or horizontal transmission, independent of host sexual activity. We here demonstrate the existence of an STD in the coccinellid beetle Adalia bipunctata. This species bears a parasitic mite of the genus Coccipolipus. We show that, like many other podapolipid mites, this mite is transmitted between host individuals at a high rate during copulation. It also appears to be transmitted at a low rate between non‐copulating individuals. We show that infected female A.bipunctata produced eggs at a reduced rate, and that the eggs produced by infected females have highly decreased viability. However, no effect of infection upon host longevity was observed. The results are discussed in relation to the incidences of sexually transmitted disease in invertebrates in general, the causes of disease symptoms, and the importance of this disease in the evolution of A.bipunctata.