Symbiotic associations are ubiquitous in the microbial world and have a major role in shaping the evolution of both partners. One of the most interesting mutualistic relationships exists between protozoa and methanogenic archaea in the fermentative forestomach (rumen) of ruminant animals. Methanogens reside within and on the surface of protozoa as symbionts, and interspecies hydrogen transfer is speculated to be the main driver for physical associations observed between the two groups.In silicoanalyses of several rumen methanogen genomes have previously shown that up to 5% of genes encode adhesin-like proteins, which may be central to rumen interspecies attachment. We hypothesized that adhesin-like proteins on methanogen cell surfaces facilitate attachment to protozoal hosts. Using phage display technology, we have identified a protein (Mru_1499) fromMethanobrevibacter ruminantiumM1 as an adhesin that binds to a broad range of rumen protozoa (including the generaEpidiniumandEntodinium). This unique adhesin also binds the cell surface of the bacteriumButyrivibrio proteoclasticus, suggesting a broad adhesion spectrum for this protein.