This lichen survey was commissioned by Natural Resources Wales in order to inform the future land management of Carn Owen and its surroundings. The RIGS (Regionally Important Geological Site) was the main focus area, together with an initial assessment of the wider area. An evaluation was made on saxicolous (rock dwelling) assemblages for non-montane acid rock, together with the metalliferous and aquatic assemblages using SSSI (Site of Special Scientific Interest) selection indices developed by Sanderson et al. (2018). This assessment strongly indicates that Carn Owen should be considered for SSSI designation for both the non-montane acid rock and the metalliferous lichen flora. Initial surveys by Steve Chambers and Alan Fryday indicated that the site passed the threshold for SSSI selection for non-montane acid rock and metalliferous assemblages (Chambers 2017 & 2020). However, abundance and distribution of the scoring species was unknown and it was not possible to set a SSSI boundary. Carn Owen is a candidate SSSI for its geology and the current boundary follows that of the RIGS (Regionally Important Geological Site). The lichen assemblage score for non-montane acid rock is 58. The metalliferous Habitat Index is 18. The threshold score for SSSI consideration for both indices is 10 (Sanderson et al. 2018). The Acid Watercourse Quality Index (AQUI) score is 7 (threshold =11). The non-montane acid rock score is the highest recorded in Ceredigion Area of Search, although NRW do not have data on the great majority of Ceredigion sites. However, this score exceeds many SSSI’s in Wales. Higher scores for Cadair Idris SSSI and Eryri SSSI are partly due to their far larger extent. New records for the site include: 21 Nationally Scarce, 10 Nationally Rare, 2 Near Threatened, 1 Vulnerable; 1 Endangered, 7 Data Deficient and 2 species for which the UK has an International Responsibility. Two taxa are New for Wales: Lecidea sarcogynoides (Nationally Rare, RDB Vulnerable) and the lichenicolous fungus Polycoccum microcarpum (Nationally Rare) growing on Cladonia gracilis. In total six taxa were recorded as new to Cardiganshire (VC 46) including: Micarea lignaria var. endoleuca (Nationally Scarce, International Responsibility), Rhizocarpon sublavatum (Nationally Rare) and the Nationally Rare lichenicolous fungi Sphaerellothecium araneosum on Ochrolechia androgyna & Cercidospora cladoniicola on Cladonia ciliata var. ciliata. 2nd VC 46 records were made for Fuscidea gothoburgensis (Nationally Scarce) and Lepra (Pertusaria) melanochlora (Nationally Rare, Endangered). In addition, the site supports a number of as yet possibly undescribed species (Chambers, unpublished). Nitrogenous pollutants (i.e. NOx and/or ammonia compounds and their derivatives) appear to be negatively affecting this site, from both airbourne deposition and from introduced mineral licks. It is highly recommended that important lichen species and communities are monitored, in order to help inform management decisions. Illustrated target notes detail some of the rarer species and their associated habitats. A full species list is provided in Table 1 (below), and the underlying records Page 7 will be disseminated to the National Biodiversity Network (NBN), British Lichen Society (BLS) and Local Environmental Records Centre (LERC).