An evaluation was made using the recently developed JNCC (Joint Nature Conservation Committee) indices for the selection of biological SSSI’s Sanderson et al. 2018). The non-montane acid rock (Threatened Near Threatened and Notable Species) score is 16, above the threshold score of 10 for potential SSSI designation. Mynydd Presili SSSI ranks 7th out of 15 for known acid rock lichen sites in Wales. This score is impressive considering most of the other sites are higher in elevation and much greater in area. The Near Threatened Halecania spodomela (NR) was surprisingly abundant at all four sites. Other species which were abundant at all sites include Candelariella coralliza and Lasallia pustulata, both occurring on a large number of rocks. Micarea subnigrata (NS) was also frequently encounted at most sites. Four species were recorded as new to Pembrokeshire (VC 45): Lecidea swartzioidea (NS), Miriquidica pycnocarpa f. pycnocarpa (NS), Stereocaulon vesuvianum var. nodulosum (NS) and Umbilicaria deusta (NS). Other notable species found include: Caloplaca arenaria (NS), Catillaria atomarioides (NS), Clauzadeana macula (NS), Lecidea fuliginosa (NS), Lepra (Pertusaria) excludens (NS), Lepraria borealis (NS) and P. melinodes (NS). Nitrogenous pollutants (i.e., NOx and/or ammonia compounds and their derivatives) appear to be negatively affecting this site, from both airborne deposition and from excreta from grazing animals (mostly sheep with some ponies). The Air Pollution Information System website (APIS 2022) gives readings for ammonia and nitrogen deposition levels nearly three times higher than critical load levels for the sites surveyed. This is reflected in the composition of the lichen flora and the abundance of algae and cyanobacteria on some rocks and lichen thalli. Grazing appears to be keeping vegetation from growing over rocks. However, there does appear to be a superabundance of grazers (particularly sheep), and dunging and urination on rocks and the surrounding vegetation is a concern. It is recommended that grazing levels are reduced on the rocky areas of the SSSI. There also appears to be a very low occurrence of terricolous Cladina species, with only Cladonia portentosa and C. ciliata var. tenuis being rarely encounted at these sites. Conifer regeneration is apparent at Carn Menyn and should be removed as soon as possible to prevent shading of rocks and outcrops. It is important that a program of monitoring is initiated in order to help inform management decisions and conserve the important lichen flora in this area. Illustrated target notes detail some of the rarer species and their associated habitats. A full species list has been provided in the form of a BLS spreadsheet which will be disseminated to the LeRCs and NBN. Countries/Continents: Wales/United Kingdom/Europe