The ultrastructural changes occurring in porelne epithelium across the mucogingival Junction were studied. Passing from attached gingiva to alveolar mucosa the most striking change in the deeper epithelial layers was a marked decrease in cytoplasmic tonofilaments. There was a gradual reduction in the thickness of the stratum corncum and also in the number of cells contained in this layer. Attached gingiva cells in the upper stratum spinosum and stratum corneurn displayed few interdigitations but as the junctional region was traversed interdigitating cell processes became common in these strata. Intercellular spaces became wider. There is no well defined line which represents vhe mucogingival junction. Rather there is a transitional zone where features of attached gingiva coexist with features of alveolar mucosa. These changes occurring across the mucogingival junction can be correlated with the functional requirement for attached gingiva to be relatively immoveable whilst alveolar mucosa remains flexible.