To understand the role of viruses in the marine environment, it is important to know the factors affecting their temporal distribution and the abundance of lysogens. We therefore performed a seasonal and a diel study on viral distribution in Tampa Bay, Florida, USA, and detected the abundance of lysogens and bacteriocinogens amongst marine bacterial isolates from diverse marine environments. We investigated the distribution of viruses, bacterial direct counts, chlorophyll a (chl a), salinity and temperature during a 13 mo period in the Tampa Bay estuary. The results indicated that the viral population had a strong seasonal pattern with the highest concentrations (2.0 +/- 0.8 x 10(7)) in the summer and lowest (4.8 +/- 1.4 x 10(6)) in the winter. Viral abundance was negatively correlated with salinity (r = -0.803), and positively correlated with chl a concentration (r = 0.725). A diel study in a seawater mesocosm indicated that viral abundance did not vary on a diel rhythm, but rather peaked after a maximum in bacterial abundance and chl a. Dissolved DNA concentrations displayed diel rhythmicity, suggesting that viruses were not the main source of dissolved DNA. An estimation of the percentage of the bacterial standing stock lysed by viruses based on 4 h intervals ranged from 3.0 to 53.3 % per day. Screening bacterial isolates for the presence of inducible prophages indicated that 43 % were lysogens or bacteriocinogens, suggesting that lysogeny and bacteriocinogeny are common in the marine environment.