Blooms of the red-tide ciliate Mesodinium rubrum occur every year in Southampton Water from late May to August, peaking in abundance in July. During blooms, M.rubrum concentrates in sub- surface layers during the day and and also undergoes diurnal vertical migration; this results in super- saturation in the near-surface water and significant deoxygenation at lower depths. During the period of M.rubrum bloom, the mesozooplankton community of Southampton Water is typically dominated by nauplius larvae and calanoid copepods.The true nature of M.rubrum't impact on mesozooplank- ton and fish communities is equivocal, with contrasting reports of either bloom-associated avoidance or mortality by zooplankton or fish, or both, or alternatively grazing by micro- and raesoplankton on M.rubrum. A temporal sequence of zooplankton hauls conducted between mid-May and early August indicated that vertical profiles of mesozooplankton composition and abundance typically exhibited lower near-surface densities, with numbers increasing significantly below 3 m. During blooms, this pattern occurred both inside and outside patches, and therefore the reduced near-surface zooplank- ton numbers are considered to be caused primarily by turbulent mixing rather than by the associated oxygen supersaturation induced by M.rubrum. Equally, no clear pattern of mesozooplankton density differences was observed at lower depths, associated with the low concentrations of oxygen caused by M.rubrum respiration.The results of the sampling programme undertaken show that M.rubrum does not significantly exclude zooplankton from the vicinity of patches, nor is there any associated sub- stantial increase in zooplankton numbers. It is concluded that M.rubrum blooms have no immediate significant impact on the broad structure of the mesozooplankton community within Southampton Water.