Bellstedt, Dirk U., Nagy, Béla, Merwe, P. De Wet Van Der, Cotterill, Fenton P. D., Luke, Quentin & Watters, Brian R., 2025, The description of a critically endangered new species of seasonal killifish, Nothobranchius sylvaticus (Cyprinodontiformes: Nothobranchiidae), a relict species from an East African forest refugium in south-eastern Kenya, Zootaxa 5601 (1), pp. 86-108 : 97-102 Abstract A new species, Nothobranchius sylvaticus sp. nov. is described from ephemeral swamps in open areas of the Gongoni Forest in the seasonal Mkurumudzi system in south-eastern coastal Kenya. The species occupies a unique ecological niche as it inhabits small seasonal wetland habitats situated in a relict forest ecosystem. Nothobranchius sylvaticus sp. nov. is distinguished from all other members of the genus by: iridescent light blue scales on its trunk extending to cover the entire caudal peduncle; dorsal portion of head grey with distinct red lobes along the supraorbital canal; posterior scale margins on the postorbital portion of the opercle creating two pronounced brown-red oblique bars; dorsal and anal fins golden, marked with dark grey dots and stripes proximally and medially, merging into a uniform dark grey zone distally; caudal fin dark red to red proximally, plain red medially and with a slender black distal band. Phylogenetic analysis of sequences of the mitochondrial ND2 and COI, and the nuclear Glyt, MyH6 and SNX33 genes, retrieve the new species as a phylogenetically distinct lineage, in the so-called Coastal Inland clade sister to subgenus Aphyobranchius Wildekamp, 1977 and subgenus Adiniops Myers, 1924. Increasing impacts on land and water resources will continue to cause habitat degradation in the remnant forest ecosystem, exposing the ecologically vulnerable population of this newly described species to severe threats. Such changes may prevent the species from completing all stages of its annual life cycle in its ephemeral aquatic habitats, because these killifishes depend on a clay-rich substratum for embryonic development of the buried eggs through the dry season. This new species qualifies for a conservation status of Critically Endangered. Copyright © 2025 the Author(s). Published in Zootaxa journal.