The encapsulation material surrounding the endophyte of Alnus crispa var. mollis Fern. root nodule was studied by means of a variety of staining methods in light and in electron microscopy. The enzymatic degradation of the capsular material was obtained and the degradation products were studied by thin-layer chromatography. The endophyte capsule was composed mainly of granular and fibrillar non-sulfated and de-esterified polygalacturonic acid or its salts. An endophyte pectic capsule was also demonstrated in 11 additional species of non-leguminous root nodules. Small secretory vesicles, containing electron-dense material similar to that of the endophyte capsule, occurred in free multivesicular bodies throughout the host cell cytoplasm. Ultrastructural observations are consistent with the view that these bodies migrate to the host membrane envelope surrounding the endophyte, fuse with it, and deposit the contents of the small vesicles toward the endophyte cell wall. These small secretory vesicles appeared to originate as blebs from cisternae tips; dictyosomes were abundant in the infected cortical cells. The possible significance of the pectic capsule in the host–endophyte nutritional interactions is discussed.