RESUMEN
Mermithid nematodes are considered a promising biological control agent to reduce the population density of different blood-feeding vectors, i.e. black flies (Diptera: Simuliidae), which are important pests of medical and veterinary interest worldwide. Immature larvae of black flies were collected in a rill from La Rioja (Northern Spain) in the summer of 2016. Isomermis lairdi Mondet, Poinar & Bernadou, 1977 (Nematoda: Mermithidae) was found parasitizing eleven specimens of Simulium cryophilum s.l. (Rubtsov, 1959) (prevalence of 52%), which represent the first record of this nematode for Spain and the second for Europe. The confirmation of the nematode and the black fly species was carried out by both morphological and molecular approaches using the 18S ribosomal RNA and the cytochrome c oxidase subunit I genes. Phylogenetic analyses indicated that the collected specimens were Isomermis lairdi (99.4-99.9% identity with homologues from Africa) with a sequence divergence of 0.2%. The role of Isomermis lairdi as an alternative tool in the biological control of black flies in Spain should be further explored.