Previous electrophysiological studies on bobolinks, an American migratory songbird, and on homing pigeons suggested that the skin of the upper beak may be involved in magnetic-field perception, which makes this tissue likely to contain a magnetic-field receptor. In the upper-beak skin of homing pigeons, we localised high concentrations of Fe3+, which form distinct coherent elongated structures extending up to 200 μ in length. Rather than being randomly distributed over the tissue, these structures always occur in the same skin layer, the stratum laxum of the subcutis. Using transmission electron microscopy (TEM), we identified the material as aggregates of magnetite (Fe3O4) nanocrystals, in the grain-size range of superparamagnetism at ambient temperatures. The nanocrystals (with grain-sizes between 2 and 5 nm) form densely packed, encapsulated clusters of 1 to 3 μ in diameter. It is demonstrated that such a cluster undergoes shape changes as the magnetic field changes, and thus could represent the core of a magnetic-field receptor. This interpretation is supported by the fact that the found structures are adjacent to nervous material.