Surveillance of Thamnolia vermicularis, a Vulnerable (VU) macrolichen in Wales that occurs in montane heath and grassland communities, was undertaken between 2020 and 2022 at the four locations where it has only ever been recorded in Wales. Three of the four sites were revisited between July and November 2022, and the species was not found at any of these sites. The fourth site was surveyed in 2020 as part of the survey of another species, and Thamnolia was not found. It is surmised that the loss of the macrolichen is due to a combination of heavy grazing pressure, climate change, and the excessive deposition of atmospheric nitrogen acting in tandem. Further targeted survey is suggested of suitable open habitat at the highest altitudes on the mountains where it was recorded, particularly in remaining areas of open vegetation, to enable a more definitive assessment of the status of the Welsh population