Many biological processes depend on the function of proteins that detect changes in a cell's environment and transmit the information to the cytoplasm, for example, peptide hormone receptors. In Escherichia coli this class of proteins is exemplified by the sensory transducers (also called signalling proteins or methyl-accepting chemotaxis proteins) which have a central role in mediating chemotactic behaviour. The sensory transducers are the products of four genes: tsr, tar, tap and trg. Each transducer detects changes in the environmental concentration of one or a very few attractants: Tsr, serine; Tar, aspartate and maltose; Tap, unknown; and Trg, ribose and galactose. Tsr and Tar act directly as chemoreceptors for the amino acid attractants and signal changes in their degree of occupancy to the flagellar apparatus. Detection of these changes in occupancy is made possible as the transducers are methylated at multiple glutamate residues such that their level of methylation reflects the most recent chemoeffector concentration. Biochemical and genetic information concerning the serine transducer protein has been accumulating rapidly but little is known about the structure of the molecule. We present here the nucleotide sequence of the tsr gene of E. coli; the amino acid sequence derived from it suggests that the Tsr transducer protein has a relatively simple transmembrane structure that may place limits on the mechanisms available for the transmission of sensory information into the cell.