A preliminary list of Diptera (Insecta) in Garf Raydah Nature Reserve (GRNR), Kingdom of Saudi Arabia (KSA) with remarks on their zoogeographic affinities is presented. This is the first in a series of planned faunal studies on different insect orders as an output of a project proposed to study the entire entomofauna of GRNR. A total number of 177 Diptera species belonging to 130 genera, representing 38 families has been listed. Some species have been identified only to genus and listed herein as genera that were not previously recorded from KSA or of a taxonomic or faunal importance. The list records five families, 11 genera and 31 species for the first time in KSA, namely: the families Acroceridae, Asteiidae, Chamaemyiidae, Heleomyzidae and Pipunculidae; the genera Acrocera (Acroceridae), Eremisca (Asilidae), Asteia (Asteiidae), Chamaemyia sp. (Chamaemyiidae), Ochthera (Ephydridae), Physoconops (Conopidae), Pseudoleria (Heleo-myzidae), Salentia (Therevidae), Sapromyza (Lauxaniidae), Sternobrithes (Stratiomyidae), Tomosvaryella (Pipunculidae); and the species Apoclea algira (Linnaeus), Apoclea femoralis (Wiedemann), Promachus rectangularis Loew, Eremisca sp. and Oligopogon nitidus Efflatoun [Asilidae]; Phycus sp., Salentia sp. and Thereva nobilitata (Fabricius) [Therevidae]; Drapetis flavipes Macquartand Platypalpus flavicornis (Meigen) [Hybotidae]; Acrocera sp.1 and Acrocera sp.2 [Acroceridae]; Aspida-cantha atra Kertesz and Sternobrithes sp. [Stratiomyidae]; Tomos-varyella sp.1 and Tomosvaryella sp.2 [Pipunculidae]; Eristalinus taeniops (Wiedemann) and Eumerus amoenus Loew [Syrphidae]; Asteia sp. [Asteiidae]; Chamaemyia sp. [Chamaemyiidae]; Physoconops sp., and Thecophora fulvipes Rubineau-Desvoidy [Conopidae]; Ochthera sp. [Ephydridae]; Pseudoleria pectinata (Loew) [Heleomyzidae]; Cestrotus megacephalus Loew and Sapromyza sp. [Lauxaniidae]; Campylocera oculata Hendel [Pyrgotidae]; Australosepsis niveipennis (Becker) [Sepsidae]; Metasphenisca transilis Munro [Tephritidae]; Anthomyia xanthopus (Hennig) [Anthomyiidae]; and Sarcophaga inaequalis Austen [Sarcophagidae]. More than 50% of the recorded fly species are of an Afrotropical affinity, whereas the Palaearctic elements did not exceed 20%. Only 7% of the species were of both Afrotropical and Palaearctic affinities, and Oriental elements were close to nil. This emphasizes the fact that the southwestern part of KSA, including the study area, belonging to Asir Province, ought to be part of the Afrotropical Region rather than the the Palaearctic Region.