<p>Flies are medically and veterinary-important arthropods that not only transmit various pathogens to humans and animals, but are also the main agents of human and animal myiasis. The aim of this study was to determine the biodiversity and abundance of medically important flies in Tabriz County, northwestern Iran. Adult and larval specimens were collected monthly from July to October 2021 and subsequently were identified using valid morphological identification keys. Several biodiversity indices, including species richness, species evenness, Shannon–Wiener index, and degree of occurrence, were calculated. Totally, 17 species belonging to five families including Muscidae, Calliphoridae, Sarcophagidae, Fanniidae, and Anthomyiidae were captured. The highest rate of species richness (S = 14) was observed in August and September, and the lowest one (9) in October. The highest rates of species evenness and Shannon–Wiener indices were observed in October. <i>Musca domestica, Muscina stabulans, Lucilia sericata, Chrysomya albiceps, Sarcophaga africa, Sarcophaga argyrostoma, Sarcophaga aegyptica, Sarcophaga</i> sp.<i>,</i> and <i>Fannia</i> sp. were constant species while<i> Musca autumnalis, Sarcophaga lehmanni, Sarcophaga</i> (<i>Liosarcophaga</i>) sp., <i>Anthomyia</i> sp., and <i>Phaonia</i> sp. were classified as accidental species in the study area. Notably, sixteen species of necrophagous flies were reported for the first time in this region. Considering the medical and veterinary importance of flies, similar studies are recommended in other parts of the country.</p> Graphical Abstract <p></p>