Aims: To see the possibility of particle size distribution analyser (PSDA) in detecting concentration of lactobacillus contaminants in yeast fermentation. Methods and Results: A PSDA was used to rapidly determine the size and concentration of lactobacillus and Saccharomyces cerevisiae. Data showed that the aerodynamic diameters of Lactobacillus casei and S. cerevisiae cells were around 0 center dot 63 and 2 center dot 9 mu m, respectively, with both cultures showing a linear relationship between cell density and particle count on a size distribution curve of PSDA. In addition, Lactobacillus fermentum showed high similarity in bacterial size distribution and particle count numbers with L. casei. The PSDA also rapidly detected (within 1 min) the cell concentrations of S. cerevisiae and L. casei in a mixed sample with different concentration ratios with 107-109 cells ml-1 of detection range. Conclusions: PSDA was demonstrated to be useful for the rapid detection of lactobacillus and S. cerevisiae concentrations. Significance and Impact of the Study: This is the first report concerning PSDA to detect the concentration of bacteria and yeast. This method can be useful in the actual field during ethanol fermentation because of relatively easy handling and rapid detection.