Lyme borreliosis (LB) is certainly the most common infection transmitted through the bite of Ixodes in Northern Hemisphere. These ticks are also able to transmit other microorganisms such as the bacteria Anaplasma phagocytophilum (Ap) and Bartonella henselae (Bh), with the latter discovered fairly recently, leading to diferent clinical presentations often close to those of LB. The aim of this study was to evaluate the frequency of co-infection by either of these bacteria in patients with LB, particularly when a treatment with beta-lactam antibiotic was only partially effective. Of these patients, on the basis of serological data, 8.07% were simultaneously contaminated by Bh, 6.83% by Ap and 4.96% were co-infected by Bh and Ap. Since the choice of an antibiotic should take into account the specificities of these germs and especially their intracellular proliferation, these results should be considered in selecting treatment.