A thermophilic Bacillus sp. strain 2-6 was used in completely open repeated batch fermentation for producing optically pure L-lactic acid. Up to 107 g l(-1) L-lactic acid of optical purity 99.8% was obtained with NaOH as pH regulator. Unexpectedly, accumulated cells did not necessarily lead to increased L-lactic acid production. Kinetic and viable cells analyses revealed that L-lactic acid production by Bacillus sp. 2-6 was closely related to the growth of viable cells. Accumulated unviable cells had a slight or negative effect on production efficiency. Therefore, the number of repeated batches should be limited to avoid the accumulation of unviable cells. In this study, repeated open operation was shown to be feasible for optically pure L-lactic acid production, and new light shed on the roles of viable and unviable cells in repeated batch fermentation. (C) 2010 Elsevier Ltd. All rights reserved.