It has been speculated that horizontal gene transfer might be important in the evolution of strains of the marine cyanobacterium Synechococcus and that phages might mediate this process, but until now there has been no direct evidence to support this idea. We have rigorously purified bacteriophages (cyanomyoviruses) from their Synechococcus host and performed a series of experiments on phage-encapsidated DNA to reveal the presence of chromosomal Synechococcus DNA. Quantitative polymerase chain reaction has shown that approximately 1 in 10(5)Synechococcus phage particles contain a host marker gene in their capsids. This is the first study that has shown that phages infecting marine Synechococcus strains can package host DNA and this provides evidence for the potential importance of these phage in horizontal gene transfer.