The unicellular cyanobacterium Synechococcus PCC6301 lacks a hybridisable homologue of the strongly conserved gdhA gene of E. coli that encodes NADP-specific glutamate dehydrogenase. This is consistent with the failure to find this enzyme in extracts of the cyanobacterium. The E. coli gdhA gene was transferred to Synechococcus PCC6301 by transformation with an integrative vector. High levels of glutamate dehydrogenase activity, similar to those found in ammonium grown E. coli cells, were found in these transformants. These transformed cyanobacteria displayed an ammonium tolerant phenotype, consistent with the action of their acquired glutamate dehydrogenase activity as an ammonium detoxification mechanism. Minor differences in colony size and in growth at low light intensity were also observed.