Diffusing wave spectroscopy (DWS) and rheology have been used to study the aggregation of casein micelles dispersions induced by the addition of rennet, a proteolytic enzyme preparation. Aggregation times measured by DWS and by rheology were in good agreement. In addition, prior to the onset of aggregation DWS was able to detect the change in the casein micelle mobility due to the cleavage of kappa-casein by the enzyme. We believe that, due to its the sensitivity to changes brought in particle size, in addition to being a non-invasive and a non-intrusive technique, DWS has a potential to be implemented and used in an industrial environment, such as manufacture of yogurts and cheeses. (C) 2003 Elsevier B.V. All rights reserved.