With the use of scanning tunneling microscopy, it is shown that germanium atoms adsorbed on the (100) surface of silicon near room temperature form chainlike structures that are tilted from the substrate dimer bond direction and that consist of two-atom units arranged in adjoining substrate troughs, These units are distinctly different from surface dimers. They may provide the link missing in our understanding of the elementary processes in epitaxial film growth : the step between monomer adsorption and the initial formation of two-dimensional growth islands.