Ammonium transport was examined in the symbiotic nitrogenfixing acterium Rhizobium meliloti. This was done by assaying for the uptake of a radio-labelled analogue of ammonium, 14C-methylamine. The activity was shown to vary depending on the nitrogen source and its concentration m the growth medium, and to be inducible. The effect of various compounds on the actual assay itself was also assessed. Ammonium, methylamme and glutamine were found to inhibit uptake, as did sodium azide, indicating that it was an energy-requiring process. Several mutants in nitrogen assimilation were assayed for uptake, and this showed the activity to be dependent on the sigma factor a54 and its associated activator protein NtrC, but not on the three genes for glutamine synthetase found m R . meliloti. Genes transcribed by a54 show a highly conserved promoter sequence m the -24/-12 region. A primer designed to this sequence allowed use of the PCR technique to amplify fragments containing this sequence from a gene bank of R . meliloti. These PCR products were then cloned, and eight transformants with different sized inserts selected for sequencing. Analysis of the sequence data showed one to contain the start of the m f H gene, known to be (independent, and thereby validating this method for the isolation of a group of promoter-specific genes.