The tct (tricarboxylate transport) locus of Salmonella typhimurium is found at 59 units between nalB and pheA (Somers et al. 1981). This locus was further resolved by fine structure genetic mapping and by analysis of one of the gene products, the tricarboxylate binding protein (C protein). 135 independent fluorocitrate resistant clones were isolated and 12 point mutants were ordered by 3 point reciprocal crosses using an adjacent Tn10 insertion. Eight spontaneous deletions as well as 17 deletions arising from imprecise excisions of internal and flanking Tn10 elements were used to construct a deletion map comprising 21 deletion segments. 115 strains were than assigned to these segments to complete the fine-structure map. Using the expression of the C protein as a guide, an analysis of a variety of mutant strains indicated: that the tct locus is composed of at least four genes and transcription is clockwise; the C protein structural gene (tctC) resides in the centre of the region and codes for two isoelectric forms of the C gene product; tctC is flanked by two regions which are involved in transport but whose gene products are not yet identified.