This thesis aims to investigate how SRB endure long periods of nutrient deprivation in the oligotrophic conditions of the North Sea. The presence of small cells in the marine environment has been extensively documented. These small cells are termed ultramicrobacteria, and are defined as being less than 0.3 μm in diameter. The formation of small cells by SRB was postulated to facilitate penetration of SRB deep within oil reservoirs, during water injection, exacerbating SRB associated problems. These studies revealed that a maximum of 15% of starving SRB populations formed UMB. Cultures starved for up to 6 years did not demonstrate an increase in UMB formation. Cell size studies revealed that SRB demonstrated a maximum 62% cell size decrease during starvation. Total cell counts revealed a constant cell number throughout starvation studies indicating a decrease in cell size by cell dwarfing. Transmission electron microscopy revealed a decrease in cellular content during starvation. This is consistent with a decrease in cell diameter during starvation. There was no difference in cell size decrease when cells were starved in the presence or absence of sulphate. There appeared, however, to be enhanced recoverability of cells starved in the presence of sulphate. SRB were demonstrated to be able to withstand simultaneous periods of sulphate and carbon starvation. This may have serious consequences for the oil industry as sulphate is often limiting in oil reservoirs. This evidence suggests that SRB could endure such conditions and recover when sulphate becomes available. SRB appear to enter a dormant phase shortly after the onset of starvation. Metabolic studies indicated that the entry into starvation was characterised by an initial increase in metabolic activity followed by a sharp decrease in metabolic activity to negligible levels. Metabolic activity could be re-initiated following inoculation into fresh growth medium.