Mineralized collagen fibrils constitute a basic structural unit of collagenous mineralized tissues such as dentin and bone. Understanding of the mechanisms of collagen mineralization is vital for development of new materials for the hard tissue repair. We carried out bioinspired mineralization of reconstituted collagen fibrils using poly L-aspartic acid, as an analogue of noncollagenous acidic proteins. Transmission electron microscopy and electron diffraction studies of the reaction products revealed stacks of ribbon-shaped apatitic crystals, deposited within the fibrils with their c-axes coaligned with the fibril axes. Such structural organization closely resembles mineralized collagen of bone and dentin. Initial mineral deposits formed in the fibrils lacked a long-range crystallographic order and transformed into crystals with time. Interestingly, the shape and organization of these amorphous deposits were similar to the crystals found in the mature mineralized fibrils. We demonstrate that the interactions between collagen and poly L-aspartic acid are essential for the mineralized collagen fibrils formation, while collagen alone does not affect mineral formation and poly L-aspartic acid inhibits mineralization in a concentration dependent manner. These results provide new insights into basic mechanisms of collagen mineralization and can lead to the development of novel bioinspired nanostructured materials. © 2008 American Chemical Society.