Omega-3 fatty acids (omega 3 FAs) have attracted attention because they have various health benefits for humans. Fish oils are currently major sources of omega 3 FAs, but a sustainable supply of omega 3 FAs based on fish oils is problematic because of the increasing demand. In this study, the production potential of a genetically engineered marine cyanobacterium, Synechococcus sp. strain NKBG 15041c, was examined as an alternative source of omega 3 FAs. A change in fatty acid composition of this cyanobacterium was successfully induced by the expression of a heterologous Delta 6-desaturase, and the transformants synthesized stearidonic acid, which the wild type cannot produce. As a result of optimization of culture conditions, maximal contents of stearidonic acid and total omega 3 FAs reached 12.2 +/- 2.4 and 118.1 +/- 3.5 mg/g, respectively. The maximal omega 3 FA productivity was 4.6 +/- 0.7 mg/(Lai...day). These are the highest values of the contents of stearidonic acid and omega 3 FAs in genetically engineered cyanobacteria reported thus far. Therefore, genetically engineered Synechococcus sp. strain NKBG 15041c may be a promising sustainable source of omega 3 fatty acids.