The ultrastructural differentiation of mycorrhizas of the achlorophyllous plant Monotropa hypopitys was examined in relation to the stages of growth of the shoot. Plants were collected at regular intervals during the growing season and their roots were immediately excised and prepared for scanning and transmission electron microscopy. In addition to the presence of a fungal sheath and Hartig net of similar structure to those seen in ectomycorrhizas, the outer cortical cells frequently contain characteristic fungal intrusions. While not breaching the host cell wall, these intrusions stimulate its invagination. The resulting structures, analogous to the transfer cells found in regions of intensive solute mobilization and transport, are produced in greatest numbers during the period of most rapid shoot extension. The relationship between their structure and possible function is discussed. The structure of mycorrhizas in Monotropa hypopitys is sufficiently distinct from that of arbutoid mycorrhizas, to warrant the creation of a new category which we term ‘monotropoid’.