Assays of chitinase, cellulase, and amylase activities in the crystalline styles of selected fast- and slow-growing families of the mussel Perna canaliculus were carried out on 2 occasions over 6 mo, during which time the shellfish grew in a normal mussel cultivation situation. The data did not support the hypothesis that differences in growth rate by these families could be related to differences in the expression of digestive enzymes in the styles, leading to improved nutritional efficiency by faster growing families. On the contrary, the styles of faster growing families in general exhibited lower rates of enzyme activity. Differences in enzyme activities between families appeared to be related to the size of the individuals rather than their genetic heritage. The crystalline styles of larger, more mature shellfish contained less moisture and less soluble protein, and the lower rates of enzyme activity in larger individuals may be indicative of a reallocation of style protein synthesis toward higher levels of insoluble structural proteins.