The distribution of DNA values in the cortex of mycorrhizal and non-mycorrhizal roots of Dactylorhiza purpurella and Spathoglottis plicata has been studied by Feulgen microdensitometry. Endoreduplication occurs during differentiation of cortical parenchyma. Nuclei with two or four times the normal DNA content are present in infected and adjacent uninfected cells. Autoradiography following (3)H-thymidine incorporation showed that DNA synthesis is induced in fully differentiated cortical cells of S. plicata during infection by Tulasnella calospora.