The unicellular cyanobacterial species Synechococcus and Prochlorococcus are known to be vital components of marine ecosystems, especially in the vast oligotrophic areas. Lytic cyanophages infecting unicellular phytoplankton are prevalent and have been demonstrated to act as important constraints on community composition contributing to the seasonal succession in genotypes. Lysogeny in Synechococcus has been documented experimentally in natural environments by prophage induction. At this time it is completely unknown how prevalent lysogeny is among Synechococcus populations. This study was performed to document important features such as size, morphology and the incidence of the T4-like capsid portal protein gene (g20) in a group of lytic Synechococcus cyanophages (35 isolates) isolated from the Gulf of Mexico. A group of Synechococcus isolates (24 strains) were isolated concurrently to investigate the virulence and cross-infectivity of the lytic cyanophages and to determine the frequency of lysogeny by detection of inducible prophage. The host range of the cyanophages toward these Synechococcus strains ranged from 1 of 25 (host of isolation only) to 17 of 25 (68%). Of the 35 cyanophage isolates the large majority were myoviruses (94%) and only two (6%) were of the podovirus type. The expected polymerase chain reaction product for g20 was detected in 20 of the phages (63%). The presence of a detectable g20 was associated with low-infectivity cyanophages at the 90% confidence interval. The Synechococcus strains varied in their resistance to lytic infection from 11% to resistance to all of the phage isolates utilized in testing. The prevalence of inducible prophage-like particles was determined in the Synechococcus strains using mitomycin C and enumerating viruses by epifluorescence microscopy. A statistically significant increase in viruses was detected in 11 of the strains (46%) in response to mitomycin C. There was no observed relationship between the occurrence of prophage induction in the Synechococcus isolates and their resistance to lytic infection. One putative lysogen was induced by continuous high light and contained a prophage-like particle with a single-stranded DNA (ssDNA) genome. Such a prophage-like particle is unlike any prophage described to date, implying that the process of lysogeny is unique in certain marine Synechococcus strains.