Seeds of Dactylorhiza majalis (Rchb. f.) Hunt & Summerhayes were sown in vitro with a compatible fungus. A mycorrhiza was established after infection through the rhizoids of the germinating seedling. Infection through the suspensor did occur, but these hyphae did not form pelotons, and embryo cells in contact with the hyphae were filled with a tannin‐like substance. Germination, and establishment of mycorrhiza, took about 14 days in vitro. From day 9 the protein reserves were hydrolysed, the protein vacuoles coalesced and starch accumulated in plastids. Certain epidermal cells developed nuclei about eight times original volume and produced rhizoids which emerged from day 11. After infection the hyphae formed pelotons in central cells with enlarged nuclei (16–64 times original volume). The intracellular hyphae developed close contacts with the hypertrophied host nuclei. Collapsed pelotons could be observed in cells from day 12, one day after infection. Meristematic activity in the uninfected chalazal end of the seedling began on day 12. On day 28 the first vascular tissue started to develop and on day 35 the beginning of a leafy shoot could be detected.