When fresh whole leaves of six different species of forage legumes were suspended in an artificial rumen medium and inoculated with rumen bacteria, bacterial adhesion and proliferation were noted at the stomata, and penetration of the stomate by these bacteria was documented by electron microscopy. The invading bacteria adhered to surfaces within the intercellular space of the leaf and produced very extensive exopolysaccharide-enclosed microcolonies. After some of the legume leaf cell walls were disorganized and ruptured by bacterial digestion, these cells (notably, parenchyma and epidermal cells) were invaded by bacteria, with subsequent formation of intracellular microcolonies. However, other cells were neither ruptured nor colonized (notably, stomata guard cells and vascular tissue). At all stages of the digestion of intact legume leaves, the rumen bacteria grew in microcolonies composed of cells of single or mixed morphological types, and a particular ecological niche was often completely and consistently occupied by a very large microcolony of cells of single or mixed morphological types.