Since 1987 we have been studying on Lyme disease in China. There were 71 isolates of Borrelia burgdorferi recovered from six species of ticks including Ixodes persulcatus, Ixodes granulatus, ixodes rangtangensis, Haemaphysalis concinna. Haemaphysalis bispinosa and Haemaphysalis longicornis, and 55 isolates from Ixodes persulcatus collected from Heilongjiang, Jilin, Liaoning, Nei-Monggol, Hebei and Xinjiang region (province). Ixodes persulcarus is the dominant species accounted for more than 80% of the total number of collected ticks, 20 or 45 per cent adult Ixodes persulcatus contained spirochetes as determined by direct immunofluorescence in the northeast and northwest. The seasonal change of adult Ixodes persulcatus is coincident with that of the patient with Erythema migrant. These studies proved that Ixodes persulcatus play the leading role in transmission of Borrelia burgdorferi to human in north region of China. It is not clear what species of tick is the principle vector in the south region of China, though the spirochetal isolates were cultivated from Haemaphysalis bispinosa, Ixodes granulatus and Ixodes rangtangensis. Comparison between Chinese strains of spirochetes from ticks and American strain B31 in Ultrastructure, monoclonal antibodies reaction and outer surface protein profile revealed certain differences.