Growth and grazing experiments with free-living bacteria and nano- and microzooplankton were conducted in February–March 1987 in the central Red Sea and the Gulf of Aden using metabolic inhibitors. Results demonstrate the existence of a dynamic 'microbial loop' which dominates the pelagic system in terms of biomass and production. In the epipelagial, bacterial abundance ranged from 5.2 to 8.8 × 10⁵ cells ml⁻¹, and heterotrophic nanoflagellate (HNF) abundance from 0.6 to 1.2 × 10³ cells ml⁻¹. Bacterial growth rates varied between 0.014 and 0.097 h⁻¹, grazing rates between 0.010 and 0.108 h⁻¹. Population grazing rates corresponded to 2.0 to 5.4 × 10⁴ bacteria consumed ml⁻¹ h⁻¹. Bacterial biomass, growth and grazing rates were higher in the central Red Sea than in the Gulf of Aden. Bacterial growth was inversely related to grazing pressure. HNF were the major bacterial consumers with average ingestion rates of 29 bacteria HNF⁻¹ h⁻¹. HNF production was controlled by microzooplankton predation. Net growth rates of bacteria and HNF were also negatively coupled and exhibited considerable temporal variation. There was also some indication for diurnal rhythms of bacterial growth and grazing loss rates. At night, grazing impact on bacteria was reduced to less than half the noon value. The temporal variations observed caution against extrapolations derived from routine experiments run at fixed hours.