Spontaneous resistance to (6S)-6-fluoro-shikimic acid arose in Escherichia coli and other enterobacteria at high frequencies, between 10(-5) and 10(-4). Two resistant variants of E. coli were tested for their susceptibilities to the diastereomeric compound, (6R)-6-fluoro-shikimate, and both of them had become resistant to this compound as well. (6S)-6-Fluoro-shikimate-resistant variants of E. coli generally failed to transport [14C]shikimate. In E. coli K-12, (6S)-6-fluoro-shikimate resistance cotransduced with his at the same frequency as shiA, a gene locus that governs shikimate transport phenotypes. We propose that the loss of susceptibility to (6S)-6-fluoro-shikimic acid in spontaneous resistant variants is due to the loss of activity of the transport system by which it enters the bacterial cytoplasm.