In most cases, mixing in brewing fermentation vessels depends principally on the liquid circulation produced by the upward movement of carbon dioxide bubbles (CO(2)-driven mixing). In other work in this area, dimensions of vessels and impellers or extent of mixing beyond impeller speed are not fully reported, making comparisons difficult. Mixing, especially between different configurations and sizes of equipment is often satisfactorily quantified in terms of power input per unit volume (kW/m(^3)). Since unstirred fermentations rely principally on the metabolism of yeast cells and associated CO(2) release to obtain mixing, the power input varies throughout the course of the fermentation. This in turn may lead to undesirable effects such as excessive lag at the start of fermentation, premature settling of yeast cells and even stuck or stopped fermentations. In experiments using a lager strain of yeast, Saccharomyces cerevisiae NCYC 1324, and a standard lager wort, it was found that the extent of mixing (power input) was a limiting factor in such fermentations. Small-scale (500 ml) fermentations were done in which mixing was applied with standard, Rushton-type impellers. Mixing (power input) was constant within individual fermentations, but was varied over the range (0 - 0.287 kW/m(^3)) for replicate fermentations. Standard fermentation parameters (gravity, dry cell weight, fermentable sugars, and flavour compounds) were monitored and it was found that there was a threshold for power input (0.036 kW/m(^3)) below which there was no difference between stirred and unstirred (control) fermentations. Above the threshold however, it was found that fermentation rate increased and hence time to attenuation decreased. The rates of formation and maximum concentrations of esters, higher alcohols and diketones was also affected by increased mixing: An impeller speed of 300 rpm and above caused a decrease in final concentration of esters, whereas the concentration of higher alcohols was increased. The use of a novel technique for the study of yeast cell viability, at-line multi-parameter flow cytometry, showed that mechanical mixing had a minor effect on viability. An increase from 2% dead cells at the end of control fermentations to 6% at the highest power input was observed. The implications of these results for the improvement of brewing fermentations with respect to savings in time and cost as well as increasing reproducibility between fermentations are discussed. Keywords: Brewing, flavour, flow cytometry, improvement, mixing, yeast.