A newly isolated lactic acid producing bacterium Lactobacillus rhamnosus M-23, from a rice washing drainage storage tank was found to produce L-(+)-lactic acid from a non-sterilized mixture of rice washing drainage and rice bran without any additions of nutrients under the simultaneous saccharification and fermentation (SSF) process. The maximum lactate yield attained was 59 g/l with a productivity of 1.23 g/l/h and a product optical purity of 95% corresponding to a conversion of 0.85 g of lactic acid per gram of sugar equivalent. (C) 2012, The Society for Biotechnology, Japan. All rights reserved.