Some indigenous microorganisms localize on epithelial surfaces in various areas of the digestive tracts of animals. One of these, a segmented, filamentous microbe, localizes on the epithelium in the small bowels of mice and rats. These filamentous microbes colonize mice at weaning time and persist in adult animals for at least 2 months. Results of the study of light and electron micrographs suggest that the microorganisms are procaryotic, and that they interact with small bowel epithelial cells to form an attachment site. This site consists of modified epithelial cell membrane and apical cytoplasm adjacent to the attached bacterium. The microbe fills the site with part of its first segment. This segment has a nipple-like appendage on the end inserted into the epithelial cell. The other segments, which compose the rest of the filament, are usually separated by septa. Many of the individual segments contain intrasegmental bodies that appear to be procaryotic cells. Some of these intrasegmental bodies are similar in morphology to the first segment of each filament inserted into an epithelial cell. These intracellular bodies may be components in the life cycle of the microorganism. The organism has not yet been cultured in recognizable form. Therefore, such a hypothesis cannot be proved as yet, nor can the microbe be classified with certainty. Because it localizes in an epithelial habitat in the small bowel, however, it may be a particularly important microbial type in the gastrointestinal ecosystem of laboratory rodents.