Summary: A new genus and species of anguinid nematode, Cotylenchus cleo gen. n., sp. n., was recovered from leaves of the western sword fern, Polystichum munitum , growing in rainforests in several locations of Olympic National Park, Washington State, USA. This new anguinid nematode induced distinct necrotic and chlorotic symptoms on fern leaves characterised by yellowing, light to brown areas between veins. The new species is characterised by a long and slender body, six incisures in the lateral field, robust stylet (12.0-13.5 μ m) with large and rounded knobs, pyriform to elongate and abutting basal pharyngeal bulb, and long conical tail with pointed terminus; females having posteriorly located vulva (V = 74-80%) and well-developed post-vulval uterine sac (26-59 μ m) and males having spicules 20-23 μ m long. Phylogenetic analysis of the partial 18S rRNA and the D2-D3 expansion segments of the 28S rRNA genes showed that this anguinid nematode formed a separate evolutionary lineage different from all other Anguinidae taxa. The new species was also characterised by sequencing the ITS rRNA and COI genes. [ABSTRACT FROM AUTHOR]