The coding sequence of a gene for a Pseudomonas syringae ethylene-forming enzyme was inserted at the psbAI locus in a cyanobacterium, Synechococcus elongatus PCC 7942 via rps12-mediated gene replacement. The recombinant strain photoautotrophically produced ethylene at 451 nl ml(-1) h(-1) OD730-1, but showed a depressed specific growth rate as well as a yellow-green phenotype indicating a severe metabolic stress. The rate of ethylene production in the recombinant culture decreased as a result of competition with faster growing ethylene-non-forming mutants that carried short nucleotide insertions within the coding sequence of the gene for the ethylene-forming enzyme.