In Reunion Island and throughout the world, cattle farms are heavily affected by outbreaks of Stomoxys stable flies, which have a very serious impact on the health and economic situation of these farms. In 1981, a first observation mentioned a fungus infecting stable flies in Reunion. The objectives of this study were 1) to precise the distribution of the fungus in the island's cattle farms, 2) to characterize the fungus morphologically and molecularly and 3) to carry out pathogenicity tests by experimental transmission. A field survey carried out in 2021, revealed the presence of infected stable flies in all environments and altitudes of the island. More than 80 % of the farms were positive, revealing that this fungus is responsible for an island-wide infection, potentially regulating Stomoxys populations. Moreover, up to twenty Stomoxys specimens could be found stuck to a single leaf. The fungus was also observed on other flies species (Calliphora sp., Lucilia sp.). In the laboratory, this fungus could be grown on different solid and liquid culture media. Morphological observations revealed an Entomophthorales producing round conidia with a papilla, simple conidiophores and rhizoids. Taxonomic classification based on morphological characteristics led to Batkoa apiculata. Molecular analysis, while confirming that the fungus belongs to the genus Batkoa (Humber, 1989), revealed that the most related strains are not associated with any described species or clustered with species not yet accepted such as B. australiensis. The ability of the fungus to infect adult of Stomoxys niger niger and Stomoxys calcitrans was demonstrated using a conidia shower from infected Stomoxys. To our knowledge, this is the first Batkoa disease described in natural populations of Stomoxys, remarkable for its intensity and durability. Further studies are needed to determine its potential as a biological control agent against stable flies in Réunion and elsewhere.