Responses to hyperosmotic shock in the lysine-producing mutant Brevibacterium lactofermentum NRRL B-11470 and the wild-type Corynebacterium glutamicum ATCC 13032 were studied in batch and continuous culture. The strains were chosen because they are used in commercial production of lysine and glutamic acid. Both strains, as well as the wild type of B. lactofermentum, were able to grow at high osmotic stress, at least 3 osmol/kg, from NaCl, sucrose, glutamic acid or lysine. The specific growth rate decreased in a nearly linear fashion with increasing stress. However, low stress from glutamic acid stimulated growth, especially in the wild type of B. lactofermentum. Both cell and cytoplasmic volume decreased spontaneously after hyperosmotic shock and no plasmolysis was observed. Addition of betaine stimulated the subsequent increase in the volumes. The volumes decreased linearly with increasing stress, except at low glutamic acid stress, which caused a volume increase. The respiration rate, measured as CO, evolution, decreased immediately after shock, but increased again and stabilized within 2 h. Betaine stimulated the respiration recovery rate.