Microtubule arrays were studied in Spiranthes sinensis (Pers.) Ames protocorms cultured in vitro either asymbiotically or symbiotically with the fungus Ceratobasidium cornigerum (Bourdot) Rogers by localizing β-tubulin in hand sections observed with laser scanning confocal microscopy. Cortical microtubules present in uncolonized cells disappeared when cells became colonized by the fungus. However, microtubules were observed between the hyphae forming hyphal coils (pelotons) in colonized cells. In these cells a close relationship between pelotons, microtubules and nuclei was often observed, and microtubules associated with hyphae formed a net-like structure through the pelotons. During senescence and condensation of the pelotons, microtubules were observed surrounding the pelotons, between collapsing hyphae and as circular profiles within the collapsed hyphal masses. In cells which contained completely collapsed hyphal masses and which were not recolonized, cortical microtubules reappeared and continued from the cell periphery through the hyphal remains to the collapsed hyphal masses. Microtubules associated with hyphae seemed to be involved in the condensation of hyphal masses in the host cells. Microtubules were also observed within hyphae, except in those which were collapsing or had collapsed.