Dynamics of mycorrhizal colonization of two orchid species, rhizomatous Cephalanthera longifolia and tuberous Dactylorhiza majalis, was studied during two growing seasons, with special emphasis on the occurrence of collapsed pelotons. No complete lysis of pelotons in digestion cells was found. Mycorrhizal colonization of subterranean organs of both species was observed to take place even before aerial shoots developed, indicating that colonization is not restricted to leafy season, and was constantly present throughout, the growing seasons with no distinct coincidence with flowering or fruiting time. Mycorrhizal colonization of C. longifolia was patchy and of low degree (mean = 4.4%. SE = 0.3%). It occurred mostly in the distal parts of main roots, lateral roots and/or in isolated root parts. Intensity of mycorrhizal colonization of D. majalis was generally higher (mean = 12.1%, SE = 0.7%). Tubers lacked colonization except in the root-like extensions.