Bioconversion of atmospheric carbon dioxide to ethylene was studied in a recombinant cyanobacterium. The gene for the ethylene-forming enzyme of Pseudomonas syringae pv. phaseolicola PK2 was cloned and expressed in the cyanobacterium Synechococcus PCC7942 R2-SPc by use of a shuttle vector pUC303. The ethylene-forming activity in vivo of Synechococcus PCC7942 R2-SPc that carried the gene for the ethylene-forming enzyme of P. syringae pv. phaseolicola PK2 was one-fifth of that of E. coil JM109 that harbored the same plasmid. The enzyme accounted for 0.021% by weight of the total soluble protein in Synechococcus PCC7942 R2-SPc.