Anastrepha ludens, the Mexican fruit fly or Mexfly, is a species of fly of the Anastrepha genus in the Tephritidae family (fruit flies). It is closely related to the Caribbean fruit fly Anastrepha suspensa, and the papaya fruit fly Anastrepha curvicauda.

Anastrepha ludens is native to Mexico and Central America and is a major pest to citrus and mango agriculture in Mexico, Central America, and the lower Rio Grande Valley. The species exhibits high fecundity and relatively long lifespans compared to other species of fruit flies. These qualities make the Mexican fruit fly a particularly aggressive invasive species, especially threatening agriculture because the larvae grow and feed on many different species of fruit. The Anastrepha genus is designated as one of three genera that pose the greatest risk to American agriculture. According to the USDA, A. ludens is the only important member of the Anastrepha genus that is subtropical instead of tropical and thus has a range much further North than most Anastrepha species. Also the USDA estimates that the Mexfly causes $1.44 billion worth of damage in a 5-year time span, mostly to citrus farms. They are frequently designated as an invasive species in Southern California and Arizona and pose a serious threat to Florida's grapefruit agriculture.

Most species in the Anastrepha genus including A. ludens have a distinctive yellow and brown coloration of the body and wings. The adult A. ludens is 7–11 mm long, or slightly larger than a common house fly. They have a mesonotum that is 2.75-3.6 mm long and a wing span of 6.6-9.0 mm.

Females have a relatively long life spans of up to 11 months. Males live even longer than females, up to 16 months. Female adult A. ludens have a long ovipositor (3.35-4.7mm) and sheath relative to body size and are capable of laying more than 1,500 eggs in their lifetimes, making A. ludens highly fecund.

Anastrepha ludens have 12 chromosomes and most cells are diploid. A 2014 genetic study of A. ludens concluded that "A. ludens populations are genetically diverse with moderate levels of differentiation." They go on to attribute this genetic diversity to natural selection across the wide habitat range of the fly and to pest management practices. In addition they found high levels of inbreeding in the species.

Gut bacteria exist in A. ludens called Enterobacteriaceae (fruit fly type bacteria). Other families of bacteria have been found in Mexican fruit flies including Vibrionaceae, Bacillaceae, Micrococcaceae, and Pseudomonadaceae. The effects of these bacteria on A. ludens are not well studied but it has been proposed by M. Aluja that A. ludens regurgitate internal bacteria onto their host and use the bacterial colonies as a protein source. The gut bacteria may also play a role in digestion and detoxification of chemicals.

Larvae have an elongated cylindrical shape typical of fruit flies and are white in color. Larvae can be up to 12 mm in length. Larvae usually pupate on the ground but have also been observed to occasionally pupate inside its host fruit. Larvae determine when to exit a fruit through physical and chemical signals such as the pH of the rotting fruit and the drop of the fruit from to the ground.

Anastrepha ludens is native to Guatemala, Mexico and possibly Costa Rica. It is an invasive species to the US. The first record of these flies spotted outside of their native habitat of Mexico and Central America was in a small Texas colony in 1903. By 1927, Mexican fruit flies were infecting citrus farms in lower Rio Grande Valley of Southern Texas and threatening farmland in California, Texas, Florida, and Arizona. In 1954, the fly species had spread westward as far as Hermosillo. The flies were rare in Costa Rica until the 1990s when they suddenly appeared on citrus plants. It is unclear if the species was native to Costa Rica or migrated there from the north. It was previously believed that the species is native to Colombia because of misidentification of A. manizaliensis but it is now known that the species does not exist there.