A new species of <i>Samsoniella</i> (<i>Cordycipitaceae</i>: <i>Hypocreales</i>) on the herald moth, <i>Scoliopteryx</i> <i>libatrix</i> (<i>Erebidae</i>: <i>Noctuoidea</i>), from caves in the British Isles

<i>Samsoniella scoliopterygis</i> (<i>Cordycipitaceae</i>: <i>Hypocreales</i>), pathogenic on the herald moth <i>Scoliopteryx libatrix</i> (<i>Erebidae</i>: <i>Noctuoidea</i>), is described as a new species, based on morphological, ecological and molecular evidence. Infected specimens of this troglophilic moth were recorded from caves, as well as from a copper mine, in southern England (Devon, Norfolk), Northern Ireland (Fermanagh) and the Republic of Ireland (Sligo). The moths were found consistently on the cave floor or rock ledges, rather than on the ceiling amongst the over-wintering, healthy moth populations; producing multiple, white or non-pigmented clavae of the cordyceps-like sexual morph. Synnemata of the asexual morph occurred sporadically <i>in situ</i> but were produced consistently <i>in vitro</i>. The ecology and taxonomy are discussed in relation to previous records of entomopathogenic fungi on <i>Scoliopteryx libatrix</i> and other trogloxene moths. <b>Citation:</b> Evans HC, Fogg T, Buddie AG, Yeap YT, Araújo JPM (2025). A new species of <i>Samsoniella</i> (<i>Cordycipitaceae</i>: <i>Hypocreales</i>) on the herald moth, <i>Scoliopteryx libatrix</i> (<i>Erebidae</i>: <i>Noctuoidea</i>), from caves in the British Isles. <i>Fungal Systematics and Evolution</i> <b>15</b>: 201-223. doi: 10.3114/fuse.2025.15.09.