SUMMARY Electrophoretic analysis of the proteins extracted from the shells of the two prawns Metapenaeus affinis (M. Edw.) and Penaeus indicus (M. Edw.) disclose that each protein can be resolved into three fractions of different mobilities in the electrical field, all moving towards the anode. Chromatographic studies of these fractions indicate that they markedly differ from one another in their amino acid compositions and as such are distinct proteins. The first fraction represents the basic protein of the crustacean cuticle as it is rich in the amino acids glycine, hydroxyproline and proline. The second fraction represents the protein involved in the phenolic tanning of the cuticle as it is rich in the amino acids phenylalanine and tyrosine. The third fraction represents the protein rich in the amino acids methionine and cysteine which seems to cause the cementing of calcium ion to it so as to cause the final hardening of the crustacean cuticle by forming S-S bonds.