Infestation level of fruit flies in the boundary zone in Rio Grande do Sul, BrazilFruit flies are considered the major pests of fruit worldwide. Tefritids and lonchaeids are major pests in this group of insects. However, little information exists regarding the potential for harm of lonchaeids. The aim of this study was to identify the infestation levels of fruit flies in the boundary zone of Rio Grande do Sul with Argentina and Uruguay. The study was conducted in 2011 and 2012 in Itaqui, Quaraí, Santana do Livramento and Uruguaiana, RS. Fruits were collected from19 exotic and native plant species, and the infestation levels in pupae/kg of fruit and pupae/ fruit were calculated. The highest rate of Ceratitis capitata infestation occurred in nectarine, with 72.01 pupae/kg and 2.51 pupae/fruit. Anastrepha fraterculus showed high levels in cherry, corresponding to 57.57 pupae/kg and 0.23 pupae/fruit. Lonchaeids were represented by Neosilba and Lonchaea, infesting papaya and star fruits, respectively. In the studied municipalities of Rio Grande do Sul, the following species of fruit flies occurred: C. capitata, A. fraterculus, Neosilba zadolicha and Lonchaea sp.