In most vessels used for brewing fermentation, mixing is dependent on liquid circulation caused by rising CO(2) bubbles. As a result, poor mixing is likely to occur at the early and latter stages of fermentation, when the rate of CO(2) evolution is low. Mechanical agitation offers the possibility of reducing the mass transfer constraints between nutrients and yeast cells associated with periods of poor mixing. This in turn can lead to more rapid fermentation, allowing savings in time and costs as well as increasing reproducibility between fermentations. The introduction of mechanical mixing to yeast fermentations however, has the potential to cause damage to yeast cells through increased hydrodynamic shear stresses. The increased rate of CO(2) evolution and the area of the gas-liquid interface in a stirred fermentation may also result in damage from an elevated rate of gas bubble coalescence and bursting. Multi-parameter flow cytometry with site-selective fluorochromes was used to gain quantitative at-line information about the physiological state of cells under different mixing conditions in an aerobic, carbon-limited continuous culture. Dual staining with propidium iodide (PI) and bis-oxonol (BOX) was used to detect changes in the integrity and polarization of the cytoplasmic membrane at the individual cell level. Simultaneous detection of forward-angle and right-angle light scatter (FALS and RALS respectively) gave information about the size and granularity (opacity) of cells. Standard microbiological techniques were used in conjunction with cytometric measurements to gain further information on cell number and viability. A stirred tank reactor (STR) with 2L working volume and two flat-bladed impellers was used. Under standard conditions, power input in the range 0.04 to 5kW/m(^3), the upper range of which was far greater than expected practical operating values, was found to have little effect on either cellular morphology or physiology. This study indicates that the potentially deleterious effects of high agitation rates can be discounted within the useful operating range for brewing fermentations (<1.0 kW/m(^3)) and for propagation cultures. Additionally, the establishment of multi-parameter flow cytometry for studies of yeast gives a useful tool for investigating the behaviour of such cells in a wide range of bioprocesses including brewing, and may eventually provide a sensitive means of process control. Keywords: Agitation, brewing, flow cytometry, mixing, physiology, yeast, stress.