The fermentations of several carbohydrates (glucose, galactose and lactose) have been monitored measuring ultrasonic velocity during the growth of Lactobacillus casei in basal MRS media. Under the conditions used, this microorganism is basically homofermentative and sugars are predominantly transformed into lactic acid. A remarkable correlation was found between the ultrasonic velocity and the bacterial catabolism. These results could be used to model changes occurring during lactic acid fermentations and show the great potential of this non-invasive technique for monitoring biotechnological processes. (c) 2006 Elsevier Ltd. All rights reserved.