The ectoine synthesis genes of the moderately halophilic bacterium Halomonas elongata DSM 3043 have been precisely located in a 2.8-kb EcoRI region of a cosmid clone previously isolated (CANOVAS, D., VARGAS, C., IGLESIAS-GUERRA, F., CSONKA, L. N., RHODES, D., VENTOSA, A., NIETO, J. J.: Isolation and characterization of salt-sensitive mutants of the moderate halophile Halomonas elongata and cloning of the ectoine synthesis genes. J. Biol. Chem. 272, 25794-25801, 1997). This region was sequenced and three open reading frames were found corresponding to the genes ectA (encoding the diaminobutyric acid acetyl transferase), ectB (encoding the diaminobutyric acid aminotransferase) and ectC (encoding the ectoine synthase). These three genes were able to restore the salt tolerance of two H. elongata mutants defective in the synthesis of ectoine (strains CHR62 and CHR63). However, the H. elongata ectoine synthesis genes did not confer to Escherichia coli the ability to synthesize ectoine. Transposon insertion in the salt-sensitive mutant strain CHR63 was exactly mapped within the ectC gene. Moreover, sequences homologous to the H. elongata ect region have been found in a number of moderately halophilic bacteria belonging to the genera Halomonas and Chromohalobacter.