The successional changes in the genetic diversity of Mediterranean bacterioplankton subjected to confinement were studied in an experimental 300 1 seawater enclosure. Five samples were taken at different times and analyzed by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) fingerprinting to rapidly monitor changes in the bacterial genetic diversity. DGGE analysis clearly showed variations between the samples. Three of the five samples, with different DGGE banding patterns, were further analyzed by cloning and sequencing of 16S rRNA genes. Comparative sequence analysis indicated a shift from a mixed bacterial assemblage to a community dominated by bacteria closely affiliated to a single genus, Alteromonas. Sequences obtained at the start of the experiment were affiliated with two alpha-proteobacterial and three gamma-proteobacterial lineages known from other studies of marine picoplankton. One sequence was affiliated with the Verrucomicrobiales. After 161 h of incubation two sequences represented a gamma-proteobacterial lineage also present at 0 h, but the majority of sequences clustered around that of Alteromonas macleodii. After 281 h only the dominant Alteromonas-like bacteria and bacteria distantly related to Legionella were found by cloning and sequencing. Mortality rates of bacteria indicated that grazing was the dominant mortality process when heterotrophic protozoa were abundant. Hence, changes in the genetic diversity of bacteria were partly influenced by the differential mortality of bacterial populations during the course of incubation.