Characterization of the major proteins in the crystalline styles of Perna canaliculus and Mytilus galloprovincialis was carried out to attempt to identify the molecular basis of differences in the physical structure of the styles and their biochemical significance. The bivalve P. canaliculus has a crystalline style that is typical of those with permanent hard-type styles, whereas the style of M. galloprovincialis is typical of bivalves with transient soft-type styles. The consistency of the styles is clearly related to their respective moisture content, and both style types contain a suite of high-molecular weight (>800 kDa and ∼95–170 kDa) glycolproteins and medium-molecular weight (40–50 kDa) protein duplexes. Total carbohydrates (as determined by the phenol-sulfuric acid method) make up a small proportion of the mass of the styles. It is suggested that these 40–50 kDa proteins play a role in the formation of the protein gel that establishes the structural integrity of the style. Their solubility is minimal at pH 4.5–5.5, which corresponds broadly to the pH of the gastric fluids. The proportion of low-solubility, putative gel-forming proteins (at∼pH 5.5) is greater in the hard styles of P. canaliculus than in the soft styles of M. galloprovincialis. One- and 2-dimensional sodium dodecylsulfate—polyacrylamide gel electrophoresis and matrix-assisted laser desorption/ionization—time of flight/time of flight peptide analysis of style proteins shows that the major digestive enzymes (cellulases, amylases, laminarinases) make up only a minor proportion of total protein content of the styles. Amino acid de novo sequencing shows there is close similarity between the sequences of several peptide fragments of the prominent 40–50 kDa protein duplexes and myosinases I and II annotated within an M. galloprovincialis express sequence-tagged database (“Mytibase”). Myosinases belong to a large family of astacin-like metalloproteins that have numerous functions and are widely distributed in nature, although in many cases their role is unknown. It has yet to be determined whether these proteins have enzymatic activity; however, they are clearly important as part of the style matrix of these 2 mussels species and probably the crystalline styles of other bivalve molluscs as well.