The diversity of myxomycetes (plasmodial slime molds) associated with tropical mountain forests of Phia Oắc National Park in Northern Vietnam has been investigated during the field expedition in 2018. Surveys have been carried out in various plant communities at altitudes ranging from 1000 to 1850 m above sea level, where samples for moist chamber cultures were collected. In total, the survey included 306 records obtained from 310 moist chamber cultures prepared with samples taken from the bark surface of living trees and lianas, ground and aerial litter and coarse woody debris. There were 238 records of yielding myxomycete fructifications made. Determinations resulted in 42 taxa (41 morphospecies and 1 variety) from 17 genera and 8 families. One collection of Paradiacheopsis could not be clearly assigned to any described species. Eleven taxa were recorded for Vietnam for the first time, and all taxa were new for the nature reserve. The most significant diversity was observed for corticolous species with smal