Abstract Background: Documentation on water mites in Spain is scarce, as is information on the parasite-host relationship between certain water mite species and representatives of the dipteran family Simuliidae. The discomfort caused to humans and animals by black fies seems to be increasing in recent years. In this context, an investigation of parasitic water mites is of great importance, not only from the point of view of biodiversity, but also in terms of their potential to control black fy populations. Methods: Rivers across a wide region of eastern Spain were sampled to determine the specifc richness of simuliid dipterans and to investigate their possible parasites, such as water mites, mermithid nematodes and microsporidia (fungal microbes). Data on environmental variables, abundance, prevalence and intensity of parasitism on the collected specimens were analyzed. Results: In 10 streams, 15,396 simuliid pupae were collected and checked for the presence of water mite larvae; 426 pupae in seven streams were found to be associated with water mite larvae. Of the 21 simuliid species identifed based on morphological characters, eight were found to be associated with water mite larvae. Water mite infection was not equally distributed among black fy species. Also, the prevalence of parasitism was low and difered among simuliid species, ranging from one to 13 water mites per black fy pupa. Variation at the intra- and interspecifc levels was detected in terms of the number of water mites inside the black fy cocoons. Free-living deutonymphal and adult water mites representing 15 diferent species of six genera and fve families were morphologically identifed. The taxonomic identity of the parasitic mite larvae is unclear at present. Morphologically, they ft descriptions of larval Sperchon (Hispidosperchon) algeriensis Lundblad, 1942, but the possibility cannot be excluded that they represent Sperchon algeriensis, the most abundant species at the adult stage in this study and unknown at the larval stage, or even another species of the genus. A molecular analysis produced for the frst time cytochrome oxidase I gene sequences for S. algeriensis. Conclusions: Our results contribute to current knowledge on Spanish Hydrachnidia and their relationships with simuliids as hosts. However, further research is needed to evaluate the diversity, distribution, bioecology and prevalence of this parasitism.