The mitogen-activated protein kinase (MAPK) cascade, composed of MAPK kinase kinase (MAP3K), MAPK kinase (MAP2K), and MAPK, is abundantly conserved in all eukaryotes. MAPK along with MAPK cascade plays a vital regulatory role in the plant-arbuscular mycorrhiza/rhizobium nodule symbioses. However, the biological function of MAPK in orchid mycorrhiza (OM) symbiosis remains elusive. In the present study, a MAPK gene, designated as DoMPK1 (GenBank accession No. JX297594), was identified from D. officinale roots infected by an OM fungus-Mycena sp. using the reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE) methods. The full length cDNA of DoMPK1 was 1 263 bp and encoded a 372 aa protein with a molecular weight of 42.61 kD and an isoelectric point (pI) of 6.07. The deduced DoMPK1 protein contained the conserved serine/threonine-protein kinase catalytic domain (39-325) and MAP kinase signature (77-177). Multiple sequence alignment and phylogenetic analysis demonstrated that DoMPK1 was highly homologous (71%-85%) to MAPK genes from various plant species and was closely related to those from monocots. Real time quantitative PCR (qPCR) analysis revealed that DoMPK1 was constitutively expressed in leaves, stems, roots and seeds, and the transcript abundance was not significantly different in the four included tissues. Furthermore, DoMPK1 transcript was markedly induced in roots at 30 d after fungal infection, with 7.91 fold compared to that of the mock inoculated roots, suggesting implication of DoMPK1 in the early D. officinale and Mycena sp. interaction and an essential role in the symbiosis. Our study characterized a MAPK gene associated with OM symbiosis for the first time, and will be helpful for further functional elucidation of DoMPK1 involving in D. officinale and Mycena sp. symbiotic interaction.