More than 800 Borellia hermsii in mouse plasma were required for establishment of growth in an artificial medium (Kelly), but only a single organism of a fully adapted strain (25th subculture) was required for a successful subculture. As judged by generation time, maximal concentration in culture, and length and motility of the organism, the process of adaptation extended through at least 11 subcultures. Because the organisms regularly died shortly after the logarithmic growth phase, transfers at 7- to 10-day intervals were required to maintain continuous cultures.