In our earlier investigations it has been shown that the gastrointestinal (GI) tract of the infant mouse (4-5 days old) can be colonized following a single intragastric challenge with Candida albicans. This makes it possible to investigate the sequence of events which occur during colonization of the GI tract by this opportunistic yeast. Two strains of C. albicans, CA 30 and NS 33, which were shown in earlier studies to differ markedly in their ability to persist in the GI tract were examined. The SEM was used to reveal the location of the yeast and their structural association with the surface of tissues of the gut at early times after intragastric inoculation. Animals were sacrificed after challenge, the GI tract was removed from each mouse and subdivided into the stomach, upper intestine, mid-intestine, ileum, cecum and large bowel. The number of colony forming units was determined by homogenizing these segments and plating them out on sabouraud's dextrose agar. The microenvironment of each segment was preserved by freezing samples in liquid nitrogen prior to processing for the SEM. The distribution and level of counts of the strains studied in the GI tract were comparable during the three week period. Both strains of C. albicans associated with the secreting epithelium and the keratinized epithelium of the stomach. Yeast also associated with the mucus layer and the epithelial surface throughout the GI tract. Those yeast adhering to the epithelial surfaces of the GI tract were frequently covered by a layer of mucus which may aid in colonization.