Contact-mediated cell-cell interactions play an important role in the social life-style of Myxococcus xanthus. Previous investigations have demonstrated that fimbriae (also referred to as pili) and extracellular fibrils are involved in these social interactions (L. J. Shimkets, Microbiol. Rev. 54:473-501, 1990). We have used the relatively new technique of low-voltage scanning electron microscopy (an ultra-high-resolution scanning technique that allows for the nanometer resolution of biological materials) to observe the topological details of cell-cell interactions in M. xanthus. Our observations indicated that the fibrils (which measure approximately 30 nm in diameter) are produced most extensively by cells that are in close contact with each other and are aberrantly produced by the cohesion-deficient dsp mutants. Immunogold analysis identified an antigen which is located exclusively on the extracellular fibrils. Western blots (immunoblots) of this antigen (designated FA-1 for fibrillar antigen 1) indicated that it is composed of several immunoreactive bands (molecular size range, 90 to 14 kDa), all of which are sensitive to protease digestion. A technique for fibril isolation was developed by using FA-1 as a fibril-specific marker. Low-voltage scanning electron microscope observations of swarming cells demonstrated that the expression of fibrils is differentially regulated between adventurous (individual) and socially (group) motile cells. The differential expression of fibrils suggests the existence of a mechanism for the regulation of fibril biosynthesis that functions within the overall system governing social interactions in M. xanthus.