This thesis demonstrates that a number of lactic acid bacteria and yeasts can grow during barley tempeh fermentation with Rhizopus oligosporus without reducing the growth of R. oligosporus. The growth of these microorganisms possibly improves barley tempeh nutritional and hygienic quality. Real-time PCR can selectively quantify the growth of R. oligosporus and yeast in co-cultivation system by designing specific primers pairs. Image analysis can be used online to follow the fermentation process in industrial scale.