Langmuir monolayers have been used to study the interfacial adsorption of poly(alpha,beta-aspartic acid) peptides. At pH 5-6, isotherms for the positively-charged octadecylamine (CH3(CH2)(16)NH3+) monolayer showed expanded Limiting areas per molecule (30-35 Angstrom(2)) compared to octadecylamine spread on pure water (approximate to 20 Angstrom(2)), suggesting strong adsorption and partial intercalation of a 4.1 kDa peptide between surfactant molecules. Branched-chain peptides of different molecular masses (2 and 5 kDa) and a linear 7.1 kDa molecule had similar effects, but the alpha-aspartate monomer had no influence on the isotherms. Thus, polycarboxylate molecules were required for effective adsorption although the number of acidic groups on the polymers was not important. The presence of calcium ions in the subphase, at pH 5-6, appeared to have little effect for the branched peptides studied, but the linear 7.1 kDa poly(alpha,beta-aspartate) showed enhanced, if variable, intercalation (30-75 Angstrom(2)). This was probably a result of conformational changes rendering the molecule more predisposed for adsorption. No effects were observed with negativelycharged or neutral monolayers, indicating that the driving force for adsorption at the air/water interface was electrostatic. The presence of poly(alpha,beta-aspartate) in supersaturated calcium bicarbonate solutions resulted in vaterite nucleation rather than calcite crystallization as in control experiments. Calcium carbonate crystallization under compressed octadecylamine monolayers produced oriented vaterite crystals, whereas in the presence of the peptides, nonoriented vaterite aggregates were formed at the monolayer/solution interface.