The nucleic acid of Synechococcus sp. cyanophage was identified as double-stranded DNA by the result of digestion with enzymes such as exonucleases, DNase, and S1 nuclease, and by acridine orange staining. The cyanophage DNA was cleaved with several restriciton ehdonucleases such as ApaI, BamHI, Bg/II, HaeIII, Eco RI, HindIII, PstI, AND aPAI gave the clearest sets of bands on agarose gels and the fragment numbers for each were 12, 20, 29, 20, and 7, respectively. The sums of the size from Bam HI and PstI digestions were estimated approximately 227$pm$4 kb, which are in agreement with the result of the pulsed field gel electrphoresis. This virus is thought to have the largest genosome among those of known cyanophages, which corresponds to the largest haed ot 90 nm when compared with the head sizes of cyanophages discovered since 1963.