Heterogeneous nucleation of calcium phosphate on an 11-mercaptoundecanoic acid self-assembled monolayer (SAM) in a pseudophysiological solution was investigated by in situ and ex situ atomic force microscopy. Calcium phosphate nanodots of 5-10 nm diameter were arranged two-dimensionally in a hexagonal closely packed structure in the initial stage of nucleation (less than or equal to3 h), reflecting the molecular arrangement of a thiol layer. After this stage, the surface was covered by random nucleated calcium phosphate particles of 20-30 nm diameter. Measurement of the growth rate of a calcium phosphate layer and X-ray photoelectron spectroscopy analysis were performed and possible qualitative models of calcium phosphate growth on a SAM were presented.