The susceptibility of mobile and cystic forms of Borrelia burgdorferi to tinidazole (TZ) was examined. The minimal bactericidal concentration (MBC) of TZ against the mobile spirochetes was >128 μg/ml at 37°C in micro-oxic atmosphere when incubated for 14 days. TZ significantly reduced the conversion of mobile spirochetes to cystic forms during incubation. The MBC for older (10- months-old) cysts at 37°C in a micro-oxic atmosphere was >0.5 μg/ml, but >0.125 μg/ml for young (1-day-old) cysts. Acridine orange staining, dark-field microscopy and transmission electron microscopy revealed that, when the concentration of TZ was ≥ MBC, the contents of the cysts were partly degraded, core structures did not develop inside the young cysts, and the amount of RNA in these cysts decreased significantly. When cysts were exposed to TZ, both the spirochetal structures and core structures inside the cysts dissolved, and the production of blebs was significantly reduced. These observations may be valuable in the treatment of resistant infections caused by B. burgdorferi, and suggest that a combination of TZ and a macrolide antibiotic could eradicate both cystic and mobile forms of B. burgdorferi. [Int Microbiol 2004; 7(2):139–142]