The susceptibility of an isolate of Rochalimaea quintana, Rochalimaea vinsonii and the newly recognized Rochalimaea henselae to antimicrobial agents was determined by dilution in Mueller-Hinton agar supplemented with 5% sheep blood. Two inocula of 102 and 104 cfu were used and tests were read after incubation at 5 and 7 days at 37°C in an atmosphere enriched to 5% carbon dioxide. Each isolate was susceptible to amoxycilhin, third-generation cephalosporins, tetracycines, macrolides and rifampicin, with complete inhibition of bacterial growth at concentrations lower than 0·125 mg/L. Co-trimoxazole and aminoglycosides were also active. Conversely, MICa for oxacillin, cephalothin, clindamycin, chloramphenicol and the fluoro quinolones were around the maximum concentrations achievable in serum. Vancomycin showed moderate activity with MICa of ≤4 mg/L. These results are consistent with clinical experience in antibiotic treatment of R. henselac infections since failures were reported with first-generation cephalosporins and the isoxazolyl penicillins. However, the frequent relapses that are observed with other β-lactams, tetracyclines or macrolides do not accord with the in-vitro susceptibility of Rochalimaea spp. Poor bactericidal activity as well as the intracellular growth of the bacteria within infected hosts, may explain such discrepancies.