Photosynthetic conversion of carbon dioxide to ethylene was studied using the recombinant cyanobacterium, Synechococcus sp. strain PCC 7942 R2-SPc which expresses the ethylene-forming enzyme (EFE) from Pseudomonas syringae pv. phaseolicola PK2. The gene encoding the EFE (efe gene) from P. syringae was introduced into the cyanobacterium utilizing the pUC303 shuttle vector into which the efe gene was placed under the control of various transcriptional signals, i.e., the native promoter and terminator of the efe gene (pUC303-EFE03), the Escherichia coli lacZ promoter and the efe gene terminator (pUC303-EFE10 and pUC303-EFE30), or the promoter and terminator of the psbAI gene from Synechococcus sp. PCC7942 which codes for the D1 protein in photosystem II (pEXE3). Among these configurations, EFE activity measured in the cell-free extracts of transformants that harbored the pEXE3 was highest. However, ethylene production in vivo of the transformants carrying pEXE3 declined with the number of generations, because homologous recombination of DNA sequences on the pEXE3 plasmid and host chromosomal psbAI locus took place. Deletion of the 5＇-upstream region of the psbAI promoter and the 3＇-downstream region of the psbAT terminator in pEXE3 resulted in pEXE3 Delta 8 which showed the highest level of ethylene-forming activity, although the latter plasmid was still unstable with a half-life of only 12 generations. The amount of carbon incorporated into ethylene was calculated as a percentage of the total carbon fixed, the maximum value of which was 5.84% in the recombinant cyanobacterium harboring pUC303-EFE03.