A novel degenerated primer set was designed to amplify acyl homoserine lactone (AHL) synthetase genes from members of the family Rhizobiaceae. The primer set successfully amplified AHL synthetase genes from pure cultures of AHL producers from Rhizobiaceae, but not from AHL producers out of the Rhizobiaceae family, indicating the specificity of this primer set to the Rhizobiaceae family. An inoculation experiment showed that the minimal detectable concentration of AHL producers from the soil was around 2.5 x 10(7) CFU/g soil. When applying to environmental samples, 7 and 14 different genotypes of AHL synthetase genes were identified in the rhizosphere of Glycine max and Vigna unguiculata, respectively, which revealed complicated and unknown AHL-based quorum-sensing networks in the rhizosphere. This is the first primer set that covers diverse AHL synthetase genes from different genera. It will be a useful culture-independent approach for better understanding of the ecological significance of QS in natural habitats.