L-Serine as well as L-valine inhibits the growth of Escherichia coli cells, and L-isoleucine releases this growth inhibition. We isolated an E. coli mutant (designated as WAT9) that was able to grow on lactate (or glucose) as a carbon source even in the presence of L-serine, the parent not being able to. Cells of WAT9 were not able to grow on L-serine as a carbon source even if L-isoleucine was present in the culture medium, while the parental cells grew. This mutant was shown to lack the principal L-serine transporter in E. coli, the Na+/ serine symporter. This mutant is useful for analysis of the role(s) of the Na+/serine symporter in cell physiology and as a host for the cloning of L-serine transporter gene(s). In fact, we cloned a gene encoding a serine transporter from chromosomal DNA of E. coli using WAT9 as the host. The gene enabled the mutant cells to grow on L-serine. Transport activity for L-serine was restored in the mutant cells harboring a plasmid carrying the gene. We partially sequenced the gene and found that it was the tdcC gene. We showed that TdcC is an H+/serine symporter.