A collection of 233 rhizobia strains from 30 different sites across Saskatchewan, Canada was assayed for 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, with 27 of the strains displaying activity. When all 27 strains were characterized based on 16S rRNA gene sequences, it was noted that 26 strains are close to Rhizobium leguminosarum and one strain is close to Rhizobium gallicum. Polymerase chain reaction (PCR) was used to rapidly isolate ACC deaminase structural genes from the above-mentioned 27 strains; 17 of them have 99% identities with the previously characterized ACC deaminase structural gene (acdS) from R. leguminosarum bv. viciae 128C53K, whereas the other ten strains are 84% identical (864-866/1,020 bp) compared to the acdS from strain 128C53K. Southern hybridization showed that each strain has only one ACC deaminase gene. Using inverse PCR, the region upstream of the ACC deaminase structural genes was characterized for all 27 strains, and 17 of these strains were shown to encode a leucine-responsive regulatory protein. The results are discussed in the context of a previously proposed model for the regulation of bacterial ACC deaminase in R. leguminosarum 128C53K.