Different concentrations of soluble matrix extracts from Mytilus edulis and polyaspartic acid were mixed with CaCl2 aqueous solutions to produce CaCO3 crystallizations under controlled conditions. The obtained crystals were observed at several intervals of time by optical microscopy and scanning electron microscopy (SEM). Image treatment of the SEM images allowed quantifying the crystal size distribution (CSD) in different crystallization conditions. CSD graphics showed that polyaspartic acid induced nucleation and inhibited crystal growth, and soluble matrix extracts from Mytilus edulis induced nucleation, but its inhibition rate of crystal growth is less than polyaspartic. Soluble matrix extracts from Mytilus edidis showed an equilibrium between nucleation and crystal growth. Also, XRD patterns have been acquired in order to determine the crystalline phases obtained by inducing crystallization (calcite and vaterite). Vaterite polymorph appeared in all induced crystallization events, but not in blank aqueous solutions. (C) 2003 Elsevier B.V. All rights reserved.