Patterning in nature typically occurs through self-organization, and interest has developed recently in the use of such spontaneous processes to fabricate periodically structured materials at the nanometre scale. For example, ordered arrays of semiconductor ＇quantum dot＇ particles (superlattices) have been created by deposition from a suspension(1), or by self-organization of diffusing atoms on surfaces(2) or in sequentially grown stacked layers(3). The spontaneous formation of layered structures in epitaxial growth has also been reported, and attributed to the process of spinodal decomposition(4,5). Yet highly ordered layered superlattices, developed for applications in optoelectronics (and in future perhaps for thermoelectrics(6)), are created ＇by hand＇ through the sequential deposition of two different materials. Here we show that superlattices can appear spontaneously during crystal growth of an alloy, as appear spontaneously a consequence of the distribution of strain at surface step sites. When a strained alloy grows by ＇step flow＇, the surface steps form periodic bunches(7). We find that the resulting modulated strain field biases the incorporation of the respective alloy components at different steps in the bunch, leading to segregation and superlattice formation. We also present experimental observations (X-ray diffraction and electron microscopy) of a silicon-germanium alloy grown on silicon, which show clear evidence for the formation of such a self-organized structure.